This was an idea that I was comtemplating building a long time ago — using the activity stream as a forum.
Hard part would be sorting an activity post and its comments in a linear fashion (or flat mode) since activity comments are nested. If you like threaded comments, then you’re already set!
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The pros and cons of using the activity stream vs. forum have been highly discussed. You probably already know them!
If you want to focus on the forum in groups, then minimize the impact of the activity stream in groups. One way of doing this is by removing the activity post form so that all activity will have to run through the forum.