This isn't XMPP's or OTR's fault - from a high level to have a secure multi-party discussion, every participant must individually verify every other participant. This creates a lot of overhead with large groups. What you can do instead is to delegate your trust to, say, a key member of the group - if they trust every member, so do you. This is the model that XMPP supports. Set up a server that only accepts encrypted and securely-authenticated connections (using whichever mechanism you have the most faith in) and only grant access to the room to trusted individuals.