Since you'd be receiving vouches from all group members, you'd get some data about who trusts who's message. To the user, you'd probably highlight the message w

Since you'd be receiving vouches from all group members, you'd get some data about who trusts who's message. To the user, you'd probably highlight the message with the "something's fucky" notifier (red background? a caution symbol?) and then have the further "of X vouches you've recieved, these Y disputed the message's (content/existence)" type deal. Which lets you know that Alice and Bob are in contention, which is probably good enough for most situations. "One of these three (or more if you have multiple groups of bad faith actors) sets of users are operating in bad faith" should be plenty of information for a user to make an informed decision. Even if that decision is "wow, how did I end up in a group containing multiple groups of bad actors, I should be elsewhere".