BuddyPress.org

@jivany (“On a typical WP installation, blog posts are written by site owners and/or their chosen authors. This means only certain selected groups of people can start a new discussion point/thread. Forums typically allow everyone to start a new discussion and have their peers comment on that topic.”)

I use Marius Ooms groupblog plugin in a custom theme with P2 front-end posting. Group members are automatically added as blog authors, so they can all post on the group blog and comment on what other group members have posted, in threaded comments with hide/show toggle. They can post short Twitter-like ‘status updates’ or full blog posts.