[panda921]

Got it. I was thinking that for bigger groups, it might be hard for members to keep track of who got deleted when and by whom, so it might be easier for a deleted user to slip back in without attracting notice.

Your point that the deleted user and the server can collude to add a rando to the group seems like a bigger deal, since it would be harder to catch.

To make the same point more critically, if the members need to constantly recheck the mapping of group name to membership list (to stop server cheating), then the scheme might not be buying much.