[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().)

 [mention:b04813dcb78a48ff889e6fbdf057c6bc:e9ed411860ed4f2ba0265705b8793d05]  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
        )
      )
    )
  )
)