I agree that it would be good to do it one on one. I usually find a place in the conversation where I can say somethg like, "Yea, I'm bipolar and when that happens I _________." Something that relates to a bipolar symptom. I find it easy to bring it up like that and to do it casually. Then their reaction is theirs.

I did have a problem with a friend whose mother had been bipolar and was unmedicated and destroyed her childhood. She is part of my social group. I told most everyone else before I told her hoping that the rumor mill would at least drop a hint before I had to tell her, and also that she would see me handling life well for a while. It went well too.