I feel your pain.  It took me a while to work this out because I had to work
with 7 list boxes the first time I had to do this.
The basic concept is you need to read through the ItemsSelected collection
of the list box and build a string you can use as the Where argument of the
OpenReport method.
Here is a function that will do that.  You just pass the control to it and
it read through the list and builds part of the string:

Private Function BuildWhereCondition(strControl As String) As String
'Set up the WhereCondition Argument for the reports
Dim varItem As Variant
Dim strWhere As String
Dim ctl As Control

   Set ctl = Me.Controls(strControl)
   
   Select Case ctl.ItemsSelected.Count
       Case 0  'Include All
           strWhere = ""
       Case 1  'Only One Selected
           strWhere = "= '" & _
               ctl.ItemData(ctl.ItemsSelected(0)) & "'"
       Case Else   'Multiple Selection
           strWhere = " IN ("
           
           With ctl
               For Each varItem In .ItemsSelected
                   strWhere = strWhere & "'" & .ItemData(varItem) & "', "
               Next varItem
           End With
           strWhere = Left(strWhere, Len(strWhere) - 2) & ")"
   End Select
   
   BuildWhereCondition = strWhere
   
End Function

Here is a modified version of your code (untested) that you can try. You
will have to change SOME_FIELD to the actual name of the field you are
filtering on.

Private Sub cmdEmployeeComparison_Click()
Dim stDocName As String
Dim stLinkCriteria As String
Dim strWhere As String

   On Error GoTo Err_cmdEmployeeComparison_Click

   If ValidDates() Then
       strWhere = BuildWhere(Me.list1)
       If Len(strWhere) > 0 Then
           strWhere = "Where SOME_FIELD " & strWhere
           stDocName = "rpt_EmployeeComparison"
           DoCmd.OpenReport stDocName, acPreview, ,strWhere
       Else
           MsgBox "Select Employee for Individual Reports.", vbOKOnly,
"Employee Not Selected"
       End If
   End If

Exit_cmdEmployeeComparison_Click:
   Exit Sub

Err_cmdEmployeeComparison_Click:
   MsgBox Err.Description
   Resume Exit_cmdEmployeeComparison_Click
End Sub