Many thanks Olaf thats great, however i notice that if the user is a member of $allowusers and is not present in the $users group they are included in the output of the compare object.
Ah … ok, so I misunderstood how you wanted to compare the two lists … so you should be able to get the results you want by simply exchanging the reference object and the difference object. Regardless of that you can modify the output of Compare-Object with Select-Object before pipeing the result to Out-GridView.