Code:
$RC = $objExcel.Range("A1").End($xlDown).End($xlToRight).Select

this leads you to the last cell in your range
I presume the column is always D
You can get the row like this
 Code:
...
Dim $Row
...
$Row = $objExcel.ActiveCell.Row
...

I presume the first cell with a manager in it is always 2
So I would go from the last cell in column D to the second
Anyway, here is some code
;*************************************************************************
;  Script Name:  
;  Author:        Wim Rotty
;  Date:          26/09/2007
;  Description:   Copy and paste data from one Excel book to other books
;*************************************************************************
;Script Options
If Not @LOGONMODE
   Break On
Else
   Break Off
EndIf
Dim $RC
$RC = SetOption("Explicit", "On")
$RC = SetOption("NoMacrosInStrings", "On")
$RC = SetOption("NoVarsInStrings", "On")
If @SCRIPTEXE = "KIX32.EXE"
   $RC = SetOption("WrapAtEOL", "On")
EndIf

;Declare variables
Dim $MyTable
Dim $objExcel
Dim $xlDown, $xlToLeft, $xlToRight, $xlUp
Dim $Row, $i, $Manager, $Managers[0], $j

;Initialize variables
$MyTable = "C:\Test\Tabel.xls"
$objExcel = CreateObject("Excel.Application")
If @ERROR
   Exit @ERROR
EndIf
$xlDown = -4121
$xlToLeft = -4159
$xlToRight = -4161
$xlUp = -4162
$j = 0

;Code
$objExcel.Visible = -1
$RC = $objExcel.Workbooks.Open($MyTable)
$RC = $objExcel.Selection.Autofilter
$RC = $objExcel.Range("A1").End($xlDown).End($xlToRight).Select
$Row = $objExcel.ActiveCell.Row
For $i = $row to 2 step -1
    $Manager = $objExcel.Range("D"+$i).Text
    If AScan($Managers, $Manager) = -1
        ReDim Preserve $Managers[$j]
        $Managers[$j] = $Manager
        $j = $j + 1
    EndIf
Next
$RC = $objExcel.Range($objExcel.Selection, $objExcel.Cells(1)).Select
For Each $Manager in $Managers
    $RC = $objExcel.Selection.AutoFilter(4, $Manager)
    $RC = $objExcel.Selection.Copy
    $RC = $objExcel.Workbooks.Add
    $RC = $objExcel.ActiveSheet.Paste
    $RC = $objExcel.ActiveWindow.ActivateNext
Next
$RC = $objExcel.Selection.AutoFilter(4)

;Personal UDF Section

;UDF Section