Here is a script shamelessly ripped off of TechNet's ScriptCenter (and ported to KiX)...

Code:

$strComputer = @WKSTA
$colGroups = GetObject("WinNT://" + $strComputer)
$colGroups.Filter = "group",""
For Each $objGroup In $colGroups
    $objGroup.Name ?
    For Each $objUser in $objGroup.Members
        CHR(9) + $objUser.Name ?
    Next
Next