Making simple group management page for fun with visualforce and apex. The neat thing will be being able to add Deactivated users to groups, since this is not possible through the standard Setup GUI.
However, I have confirmed that the workaround still works, and it allows a user with permission to bypass this validation by inserting an InActive UserID in place of an Active user under the 'Selected' on the "Edit Public Group" Page. Might have to generate a URL to get this to work as intended.