When you open the Local Users and Groups snap-in, using Windows 2000, to view the members of a local group, the list is empty or just displays partial membership.
If you attempt to add the missing members, you receive:
The following error occurred while attempting to save properties for group Administrators on computer computer name: The specified account name is already a member of the local group.
If you use the net command, everything appears ok.
If the local account name is the same as the domain name, you will experience this behavior, prior to Service Pack 3.