Not a secret example by any means of the imagination, but th talk of using a comments section as a group chat reminds me of a small website I came across which did exactly that. The people running it were on free hosting (so couldn't set up a forum script), but did have comments on their pages through Disqus or something like that. So, they basically treated the comments on each page as a forum, with comment subthreads as the equivalent to topics or discussions.

It was surprisingly active too. If this was a 'typical' forum, it would have been a really active one.

But yeah, if you don't have a forum, group chat or chat room, a comments section will do in a pinch.