
Answer: I used to belong to a book discussion group (I can't even remember the name of it) and I found it pretty unsatisfying. Plus it's hard to follow, remember to check, and all that.
I belong to several list-serv type discussions for psychotherapists -- one for therapists in the East Bay Area (California) and one for those practicing Emotionally Focused Therapy. Every once in awhile we have a pretty divisive discussion, and I've actually decided to pretty much stop participating. It's amazing how mean therapists can be to each other!! (I know, that's not what you want to hear, is it?)
I am also a member of a yoga forum in which I keep a blog and we have various discussions. However, since I decided to open a yoga studio, I don't have much time for talking (or doing yoga for that matter!)
I follow about 100 blogs using Bloglines, and comment often, but that's not really a discussion group.
So not too many, maybe none by strict definition.