Here is a version that does not require you to know the distinguishedName of the group...

fnADComputerInGroup("TestGroup")
 
Function fnADComputerInGroup($sGroupDN)
    Dim $objSys,$objCom,$aMemberOf,$sMemberOf
    $objSys = CreateObject("ADSystemInfo")
    $objCom = GetObject("LDAP://"+$objSys.ComputerName)
    $aMemberOf = $objCom.GetEx("memberOf")
    For Each $sMemberOf in $aMemberOf
        If InStr($sMemberOf,"CN="+$sGroupDN+",")
            $fnADComputerInGroup = NOT 0
            Exit
        EndIf
    Next
    $fnADComputerInGroup = NOT 1
EndFunction