Private CurrGroupBy As String = String.Empty
Private ConcatVal As String = String.Empty
Public Function AggConcat(GroupBy as String, ElementVal as String) as String
    If CurrGroupBy = GroupBy Then
        ConcatVal = ConcatVal & ", " & ElementVal 
    Else
        CurrGroupBy = GroupBy 
        ConcatVal = ElementVal 
    End If
    Return ConcatVal 
End Function

=RunningValue(
     Code.AggConcat(
         Fields!YourFieldToGroupBy.Value
       , Fields!YourFieldToConcat.Value
       )
   , Last
   , "YourGroupName" 
   )