The problem I see with "subject" is that it's meaningless for defining the group. If you try to create a new group with a different subject, but with the same set of people as are in another group, you'll get kicked back to the old group, which is weird.
Discussion
Ohh.. interesting. I am rendering them as one with changing topic. For small groups, there is no need for multiple subjects in parallel (unless you want to follow an email model).
Yeah, I think it's ok as is, but the way you're surfacing topic might be confusing. We could introduce an optional thread id