Kim,

Is it "your session", "everybody's session" or does it actually belong to the 3 or 4 people who dominate it? In that they started it and are the main stays?

All sessions are different, of course, but if it was set up for everybody it's wrong for just a few people to "hog it". On the other hand, newcomers and visitors to an established session have to respect the regulars' way of doing things. It really all depends on what the set up is.

I'm not fond of "show offs" either but there's a difference between that and just playing what you know. Experienced musicians are bound to know a lot more in the way of tunes and while it's nice to encourage beginners at a session it's still good to draw material from the full range of your repertoire too.