Convore may be the best solution for right now...it's super easy to organize a public/private group of people, and the daily email digests are perfect.

When There's a clearer idea of the form the SG is going to take, BuddyPress is probably the best off the shelf solution since it's open source (free & constantly being improved) easy to set up (under 15 minutes now that it's a plugin) and easily customized with themes and plugins.

I've been using it for the past few months on client projects and it's been really awesome to work with, as well as easy to use for admins and users.

This was the first idea that popped into my head, but I have not been using it yet. My only worry is that it is still not proven reliable or stood the test of time. Its seems nice and pretty, which is great, but I wouldn't want its growing pains interfering.

A more proven alternative may be simply to use google talk (with history tracking if you want that), google groups, and (gasp) email.