I've just returned from leading a jam at Megan Lynch's adult fiddle camp. Jon Weisberger (on bass) and David Thomas (on guitar) co-lead with myself on banjo. I meant to take my camera so I could post a picture, but did I remember? Of course not.
We started out easy, with "Cripple Creek" and all the fiddles playing together, trading breaks back and forth with me and David. I even sang a verse or two. Next was a singing song, "Two-Dollar Bill," and again I had all the fiddles play at the same time and it sounded pretty good because almost all the players---I'd say there were at least eight fiddles---were going for the melody. This group was solidly intermediate. Not a one had timing problems or got lost during a break, and almost everyone could improvise by picking out the melody notes to a song.
Toward the end of the session we started talking a bit about being in a regular jam, that is, a non-learning-situation jam. One point we hammered home is that it's not cool to practice your break to a song during the song, for example, while the singer is singing, or during someone else's break. Once you get into a jam it is too late to practice! You're either going to play it well, or not, and nobody but you is going to notice or care, but your performance of your break is not going to be improved by running over it a couple of times while something else is going on. It distracts from what you should be paying attention to, which is the group and how the overall song is going.
One of the women was concerned about how to let the group, or the person leading a particular song, know that she wanted to try a break, especially if she had passed up a break on a previous song. It's all about body language. If you keep your head up and make eye contact with the person singing (or the person who kicked off the instrumental) they'll know you want to try a break. One point that Jon made was that if you're worried about being passed over, it's better to start in on a break, and then back off if you notice that someone else is also taking a lead, than to be a wallflower, always waiting to be coaxed into taking a break. That way, at least everyone will know you want a chance.
One other thing to keep in mind, though, is that if the jam is very large---say more than six or seven people---chances are that not every person will get to take a break on every song. It would take forever otherwise. And just because you don't get a break on one song, does not mean you won't get a break on any of them, as long as you do the eye-contact thing. It doesn't mean they don't think you're a good player, and it doesn't mean they don't like you. Remember, the jam is about the JAM! It's not about you. Whatever you can do to make the JAM better and and make the song sound better is the right thing to be doing.