I'm in groups of 100 "trusted" traders and I must say my trust that we kept out scammers fell drastically past 40. Nobody in the groups knows all the others and with a group of 200, not leaking information is just not limited by the cryptography. Just have one of the 200 run some sql server all connect to.
That said, having a protocol that just works the same for DM or a group of 10k would be cool.
