Barry, the dumbek (not bongo) player you mentioned was in the middle of a pissing contest with someone and everyone else in the session got punished because of it.

Any type of session needs a core of folks with big egos. Not the "I'm better than everybody else" type, but the "I'm good and I know it, but I want to hear you, too" type. A session of shy introverted people will die out in a hurry. My advice on starting a session (if you can pretend I know what I'm talking about) is to try to make sure a few very confident, assertive, but generous people are going to be there and invite everyone else.

The song circle idea idea is a good one if, like Liam said, you make people feel welcome even if they don't want to lead a song/tune. I quit going to a session because of that. As someone who didn't have a lot of confidence, it scared the hell out of me.