BuddyPress.org

I want people to be able to join and leave the group, because they will be joining and leaving many other similar groups in the community. I will then add a email notification plugin so that it becomes an email newsletter with a web archive.

These two combined sound like something that would be useful in general. For example, in facebook groups, only the moderators can send an email to everyone, but anyone can join a group and chat in the group forum.

i could use a plugin that allows only admins to email all group members, but then the forum is not a record of all previous announcements, its just people chatting.