But it is (and should be allowed). I ran it in my head and if they have to keep the members in each group rather balanced, as well as create as many groups as Mafia members are currently alive, then they can’t just run the game and send Town people every round until they win. With 12 you could do what Urist proposed (and with 11 and 10 too). But then when you’re down to 9 with 3 mafia still alive you wouldn’t populate a group with all Mafia (unless you wanted to guarantee that a Mafia member would get voted on), so you would separate one and that would give Town a chance to vote on that player. If they spread themselves equally amongst all teams then they are still open to be put up. And most importantly, by putting themselves all in the same group early and often, they open themselves up for voting later. So really silly strategy if you think about it.
Does this make sense? Can anybody check my logic?