Sort appian group members for dropdown

Where do you get this list from? The groupMembers() function can already sort your items.

When you provide paging info for groupMembers, you can additionally pass these two fields in the sortInfo and control the sorting. 

a!groupMembers(
  group: cons!GROUP,
  pagingInfo: a!pagingInfo(
    startIndex: 1,
    batchSize: 100,
    sort: {
      a!sortInfo(
      field: "groupName",
      ascending: true
    ),
    a!sortInfo(
      field: "username",
      ascending: true
    )
    }
  )
).data

you can use groupMembers() function 

a!groupMembers() Function (appian.com)

If you have a particular group then you can use the a!groupMembers() function. If its the users from the environment, you can use functions from plugins and create your dropDown. Using what Harshit has given as a sample code for a group Specific requirement.

getAllUsers() is a function to get all the users from the environment. You can get it here People Functions Plug-in

a!localVariables(
  local!users: getallusers(1, - 1),
  local!choices: a!forEach(
    items: local!users,
    expression: concat(
      user(fv!item, "firstName"),
      " ",
      user(fv!item, "lastName")
    ),
    
  ),
  a!dropdownField(
    choiceLabels: local!choices,
    choiceValues: local!users,
    placeholder: "Select User"
  )
)