Can't determine group membership for local machine

I am trying to determine is a user is a member of the Administrators group
(so I know whether to add them or not), but I can't seem to get it working.

I'm using the following code:

           DirectoryEntry group;

            group = new DirectoryEntry("WinNT://" +
Environment.MachineName+ "/" + "Administrators");
            bool retVal = (bool)group.Invoke("IsMember", new object[] {
"WinNT://" + Environment.MachineName + "/" + userName });

retVal is always false.  I've tested for every user in the group.

I am a local administrator.  Might I not have some permission that I need?

Thanks,
-Mont

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