The Association for Systematic Philosophy was a moderated discussion group back in the 90's which, although small – it never had more than about a dozen active participants – ended up having quite a few members who went on to achieve interesting things.
The member who became the most famous was Jimmy Wales, founder of Wikipedia and once listed as one of Time magazine's 100 most influential people of the year.
The moderator at the time I joined was Larry Sanger, the co-founder of Wikipedia.
Soon after, Ben Kovitz became moderator. He's the one who told Sanger about the concept of a wiki.
In addition, there was the economist Bryan Caplan, author of The Myth of the Rational Voter, called the best political book of its year by the New York Times;
Philosopher Michael Huemer, author of Ethical Intuitionism and of Skepticism and the Veil of Perception, and editor of Epistemology: Contemporary Readings;
Peter J. King, author of One Hundred Philosophers;
And Jeremy P. Anderson, former Jeopardy! contestant, voice on Porsche commercials, and also a philosopher.