the mod had the problem when he loged in normaly. i tried it with the "become this user" to log in as the mod. same result. i can remove the group membership but can't add it.
to add an user to a group i search the user in the userlist or in the user administration in the admin section, i select the user, then select edit this user, then go to userrights and there i select or deselect the group membership. i tried it the same way as mod.
when i'm admin, this works perfectly, as mod symptom as described.