This is actually a very common discussion I have with various people – group collaboration.
I use a temporary 3rd party solution on PokerDIY to try and solve this requirement– see
http://www.smart-thinker.com/Support/Forums/tabid/111/forumid/26/threadid/2990/scope/posts/Default.aspx
http://www.smart-thinker.com/Support/Forums/tabid/111/forumid/5/threadid/3644/scope/posts/Default.aspx
Bear in mind that the Forums were not built for this purpose – I would estimate about 2-3 weeks work to customize it to suit your needs, but this is an off-the-cuff guess – I have never looked at the Forum source so I don’t even know if it could be done. You could need to write the integration pieces into the module and start with a good design (which I would ask someone from the Forum module to have a look at before you start coding to make sure it is possible). Investigating the source and designing the solution will be half the work, and the coding/testing the other. It’s not a job I would undertake lightly – it’s going to be massive and tricky. Once you have built a dependency into the forum module (core or 3rd party) then your upgrade path resistance is increased and you’ll pay for this later when you upgrade.
It may be cheaper in the long run to code a custom “lite” version of the Forum to do what you want as opposed to modifying the forums. It depends on your requirements etc.