It's stupidly simple and quite clever (at the same time) - it works on the unique URL, so add it to the Group Detail page in a HTML module. Because it passes in the GroupID each URL is different and therefore a unique discussion.
There are a few limitations - I want to show it to all users but only let registered users post, which is impossible, but it's a decent halfway house. (ps, I am not endorsing it, just telling you how I use it ;)