In reply to TheGeneralist:
If I understand you correctly, then yep, sounds about right. If I'm climbing with a group and we're on our second go around on a problem, a 'new arrival' will generally be invited to jump on and try it when we've all had our second go but before any of us has a third go. (If that makes sense.)
There's no real need for a hard & fast rule if people are communicating and nobody is being a dick though. And if someone is in a rush I don't think any of us would mind them breezing through straight away. (Maybe they're nipping round flashing a whole circuit or something.)
Bouldering is supposed to be sociable after all, if you want to get hung up on whose turn it is it's probably better to stick to climbing on a rope.
Incidentally, if someone is standing shyly back (not too far back), I like to think we'd spot that most of the time and invite them to come and take their turn.
Lots of new arrivals is too many though - possibly a sign that the wall's too crowded and it's time to knock it off. FWIW, I think if you're climbing in a group and another group is on a problem, it's best to do something else until the other group moves on. (And if there's nothing else available, same again - the place is too crowded.)