Keep your eye out for an update to the old forum ad-on. This new add-on allows you to block forums participation to groups. Allows users to upload supporting files and documentation to their posts, gives you a quick look snapshot about the number of posts, replies and who was the last to post on the forum. Plus now users can start their own threads.
This new functionality should greatly enhance the community and collaboration spirit on your web site, encouraging more repeat visits. The forum posts are also now searchable to make the UX even easier. The new forum add-on is currently in testing now and should be ready by the end of the month.