[Expression evaluation error at function 'reject' [line 39]: Incorrect number of parameters for function; expected 1 parameters, but found 0 parameters.]

Your syntax for "reject()" looks correct, other than maybe some more subtle error with the local variables.

But I'm here to say you don't need it in this context.  a!forEach() can handle it on its own.  For the negative case just make it return the empty set, {}, instead of null().

(There is one corner case where if *every* entry returned by the a!forEach() statement is an empty set, you get a list of empty sets (otherwise you get a list of all the good items and empty sets are ignored in the output), so to prevent this you would also want to wrap the entire a!forEach() call in a!flatten().)

 jojog0003  Can you try this code and let me know if that works for you.

a!localVariables(
  local!group: index(
    rule!CDM2_findUserGroup(loggedInUser()),
    1,
    {}
  ),
  local!escalatedTo: if(
    a!isNullOrEmpty(local!group),
    {},
    index(
      a!queryEntity(
        entity: cons!CDM2_DSE_INQUIRIES,
        query: a!query(
          aggregation: a!queryAggregation(
            aggregationColumns: {
              a!queryAggregationColumn(field: "escalatedTo", isGrouping: true)
            }
          ),
          logicalExpression: a!queryLogicalExpression(
            operator: "AND",
            filters: {
              a!queryFilter(
                field: "groupId",
                operator: "=",
                value: local!group
              )
            },
            ignoreFiltersWithEmptyValues: false
          ),
          pagingInfo: a!pagingInfo(startIndex: 1, batchSize: - 1)
        ),
        fetchTotalCount: false
      ),
      "data",
      "escalatedTo",
      null()
    )
  ),
  local!tempTmodData:a!flatten( a!forEach( 
    items: local!escalatedTo,
    expression: if(  
      a!isNullOrEmpty(fv!item),
      {}, 
      if(
        not(isusernametaken(fv!item)),
        fv!item,
        null()
      )
    )
  )
  ),
  local!tmodData: if(  
    a!isNotNullOrEmpty(local!tempTmodData), 
    reject(fn!isnull, local!tempTmodData),  
    {}  
  ),
  local!tmodGivenNames: if(
    a!isNotNullOrEmpty(local!tmodData),
    index(
      a!queryEntity(
        entity: cons!TMODS_DSE,
        query: a!query(
          selection: a!querySelection(
            columns: {
              a!queryColumn(field: "given_name"),
              a!queryColumn(field: "preferred_given_name"),
              a!queryColumn(field: "enterprise_login_id"),

            }
          ),
          logicalExpression: a!queryLogicalExpression(
            operator: "AND",
            filters: {
              a!queryFilter(
                field: "enterprise_login_id",
                operator: "IN",
                value: upper(local!tmodData)
              )
            },
            ignoreFiltersWithEmptyValues: true
          ),
          pagingInfo: a!pagingInfo(startIndex: 1, batchSize: - 1)
        ),
        fetchTotalCount: false
      ),
      "data",
      null()
    ),
    {}
  ),  
  local!groupMembers:a!forEach(
    items:  local!escalatedTo,
    expression: if(
      a!isNullOrEmpty(fv!item),
      {},
      if(
        not(isusernametaken(fv!item)),
        {
          xid: fv!item,
          name: a!localVariables(
            local!blankTmodGivenName: displayvalue(
              upper(fv!item),
              touniformstring(
                local!tmodGivenNames.enterprise_login_id
              ),
              if(                
                /*a!isNullOrEmpty(local!tmodGivenNames.preferred_given_name),*/
                a!isNullOrEmpty(index(local!tmodGivenNames.preferred_given_name,wherecontains(upper(fv!item),touniformstring(local!tmodGivenNames.enterprise_login_id)),{})),
                local!tmodGivenNames.given_name,
                local!tmodGivenNames.preferred_given_name
              ),
              {}
            ),
            if(
              a!isNullOrEmpty(local!blankTmodGivenName),
              fv!item,
              local!blankTmodGivenName
            )
          )
        },
        {
          xid: fv!item,
          name: user(fv!item, "firstName") & " " & user(fv!item, "lastName")
        }
      )
    )
  ),  
  a!recordFilterList(
    name: "Escalated To",
    isVisible: a!isNotNullOrEmpty(local!groupMembers),
    options: a!forEach(
      items: local!groupMembers,
      expression:a!recordFilterListOption(
        id: fv!index,
        name: fv!item.name,
        filter: a!queryFilter(
          field:'recordType!{92bc5c32-2509-4558-b252-56df608deb1a}.fields.{escalatedTo}',
          operator: "=",
          value: fv!item.xid
        )
      )
    )
  )
)

It's not even necessary to do this in 2 steps.  Just replace the "null()" on line 47 with an empty set.

 Mike Schmitt We can get rid of reject.

a!localVariables(
  local!group: index(
    rule!CDM2_findUserGroup(loggedInUser()),
    1,
    {}
  ),
  local!escalatedTo: if(
    a!isNullOrEmpty(local!group),
    {},
    index(
      a!queryEntity(
        entity: cons!CDM2_DSE_INQUIRIES,
        query: a!query(
          aggregation: a!queryAggregation(
            aggregationColumns: {
              a!queryAggregationColumn(field: "escalatedTo", isGrouping: true)
            }
          ),
          logicalExpression: a!queryLogicalExpression(
            operator: "AND",
            filters: {
              a!queryFilter(
                field: "groupId",
                operator: "=",
                value: local!group
              )
            },
            ignoreFiltersWithEmptyValues: false
          ),
          pagingInfo: a!pagingInfo(startIndex: 1, batchSize: - 1)
        ),
        fetchTotalCount: false
      ),
      "data",
      "escalatedTo",
      null()
    )
  ),
  local!tmodData:a!flatten( a!forEach( 
    items: local!escalatedTo,
    expression: if(  
      a!isNullOrEmpty(fv!item),
      {}, 
      if(
        not(isusernametaken(fv!item)),
        fv!item,
        {}
      )
    )
  )
  ),
  local!tmodGivenNames: if(
    a!isNotNullOrEmpty(local!tmodData),
    index(
      a!queryEntity(
        entity: cons!TMODS_DSE,
        query: a!query(
          selection: a!querySelection(
            columns: {
              a!queryColumn(field: "given_name"),
              a!queryColumn(field: "preferred_given_name"),
              a!queryColumn(field: "enterprise_login_id"),

            }
          ),
          logicalExpression: a!queryLogicalExpression(
            operator: "AND",
            filters: {
              a!queryFilter(
                field: "enterprise_login_id",
                operator: "IN",
                value: upper(local!tmodData)
              )
            },
            ignoreFiltersWithEmptyValues: true
          ),
          pagingInfo: a!pagingInfo(startIndex: 1, batchSize: - 1)
        ),
        fetchTotalCount: false
      ),
      "data",
      null()
    ),
    {}
  ),  
  local!groupMembers:a!forEach(
    items:  local!escalatedTo,
    expression: if(
      a!isNullOrEmpty(fv!item),
      {},
      if(
        not(isusernametaken(fv!item)),
        {
          xid: fv!item,
          name: a!localVariables(
            local!blankTmodGivenName: displayvalue(
              upper(fv!item),
              touniformstring(
                local!tmodGivenNames.enterprise_login_id
              ),
              if(                
                /*a!isNullOrEmpty(local!tmodGivenNames.preferred_given_name),*/
                a!isNullOrEmpty(index(local!tmodGivenNames.preferred_given_name,wherecontains(upper(fv!item),touniformstring(local!tmodGivenNames.enterprise_login_id)),{})),
                local!tmodGivenNames.given_name,
                local!tmodGivenNames.preferred_given_name
              ),
              {}
            ),
            if(
              a!isNullOrEmpty(local!blankTmodGivenName),
              fv!item,
              local!blankTmodGivenName
            )
          )
        },
        {
          xid: fv!item,
          name: user(fv!item, "firstName") & " " & user(fv!item, "lastName")
        }
      )
    )
  ),  
  a!recordFilterList(
    name: "Escalated To",
    isVisible: a!isNotNullOrEmpty(local!groupMembers),
    options: a!forEach(
      items: local!groupMembers,
      expression:a!recordFilterListOption(
        id: fv!index,
        name: fv!item.name,
        filter: a!queryFilter(
          field:'recordType!{92bc5c32-2509-4558-b252-56df608deb1a}.fields.{escalatedTo}',
          operator: "=",
          value: fv!item.xid
        )
      )
    )
  )
)

Correct, this is the implementation I was trying to guide OP to (without necessarily just typing it out for them though, lol)

Thanks for the nudge!
I was able to reproduce the code locally to better understand it So, simply paste it 

Shubham Aware 

The above is not working . I am geeting the error :

[Expression evaluation error at function a!forEach [line 132]: Error in a!forEach() expression during iteration 1: Expression evaluation error at function a!queryFilter [line 137]: Invalid index: Cannot index property 'xid' of type String into type List of Variant].

jojog0003 
You have one more issue in your code
Try this and let me know if this works for you.

a!localVariables(
  local!group: index(
    rule!CDM2_findUserGroup(loggedInUser()),
    1,
    {}
  ),
  local!escalatedTo: if(
    a!isNullOrEmpty(local!group),
    {},
    index(
      a!queryEntity(
        entity: cons!CDM2_DSE_INQUIRIES,
        query: a!query(
          aggregation: a!queryAggregation(
            aggregationColumns: {
              a!queryAggregationColumn(field: "escalatedTo", isGrouping: true)
            }
          ),
          logicalExpression: a!queryLogicalExpression(
            operator: "AND",
            filters: {
              a!queryFilter(
                field: "groupId",
                operator: "=",
                value: local!group
              )
            },
            ignoreFiltersWithEmptyValues: false
          ),
          pagingInfo: a!pagingInfo(startIndex: 1, batchSize: - 1)
        ),
        fetchTotalCount: false
      ),
      "data",
      "escalatedTo",
      null()
    )
  ),
  local!tmodData: a!flatten(
    a!forEach(
      items: local!escalatedTo,
      expression: if(
        a!isNullOrEmpty(fv!item),
        {},
        if(
          not(isusernametaken(fv!item)),
          fv!item,
          {}
        )
      )
    )
  ),
  local!tmodGivenNames: if(
    a!isNotNullOrEmpty(local!tmodData),
    index(
      a!queryEntity(
        entity: cons!TMODS_DSE,
        query: a!query(
          selection: a!querySelection(
            columns: {
              a!queryColumn(field: "given_name"),
              a!queryColumn(field: "preferred_given_name"),
              a!queryColumn(field: "enterprise_login_id"),
              
            }
          ),
          logicalExpression: a!queryLogicalExpression(
            operator: "AND",
            filters: {
              a!queryFilter(
                field: "enterprise_login_id",
                operator: "IN",
                value: upper(local!tmodData)
              )
            },
            ignoreFiltersWithEmptyValues: true
          ),
          pagingInfo: a!pagingInfo(startIndex: 1, batchSize: - 1)
        ),
        fetchTotalCount: false
      ),
      "data",
      null()
    ),
    {}
  ),
  local!groupMembers: a!forEach(
    items: local!escalatedTo,
    expression: if(
      a!isNullOrEmpty(fv!item),
      { xid: null, name: null },
      if(
        not(isusernametaken(fv!item)),
        {
          xid: fv!item,
          name: a!localVariables(
            local!blankTmodGivenName: displayvalue(
              upper(fv!item),
              touniformstring(
                local!tmodGivenNames.enterprise_login_id
              ),
              if(
                /*a!isNullOrEmpty(local!tmodGivenNames.preferred_given_name),*/
                a!isNullOrEmpty(
                  index(
                    local!tmodGivenNames.preferred_given_name,
                    wherecontains(
                      upper(fv!item),
                      touniformstring(
                        local!tmodGivenNames.enterprise_login_id
                      )
                    ),
                    {}
                  )
                ),
                local!tmodGivenNames.given_name,
                local!tmodGivenNames.preferred_given_name
              ),
              {}
            ),
            if(
              a!isNullOrEmpty(local!blankTmodGivenName),
              fv!item,
              local!blankTmodGivenName
            )
          )
        },
        {
          xid: fv!item,
          name: user(fv!item, "firstName") & " " & user(fv!item, "lastName")
        }
      )
    )
  ),
  a!recordFilterList(
    name: "Escalated To",
    isVisible: a!isNotNullOrEmpty(local!groupMembers),
    options: a!forEach(
      items: local!groupMembers,
      expression: a!recordFilterListOption(
        id: fv!index,
        name: fv!item.name,
        filter: a!queryFilter(
          field: 'recordType!{92bc5c32-2509-4558-b252-56df608deb1a}.fields.{escalatedTo}',
          operator: "=",
          value: fv!item.xid
        )
      )
    )
  )
)

Shubham Aware 

The above code is working fine but if "escaltedto" field is null then the filter also show the null dropdown like:

That is expected right?
As you already filtered the list by createdDate so might be for that range there won't be any "escalatedTo".

Can you specify your exact requirement here.