I'd say that at a session or song circle everyone is pretty much entitled to play along as they choose unless the one calling the tune has indicated otherwise. One way to compete is to cut the others out by playing something they probably won't know or in a key they can't get or start before giving them a chance to retune and re-capo.

I don't get to many Irish jams anymore but when I did, I loved the uillean pipes and the pennywhistles. They didn't ever have to be quiet for me. I don't recall ever playing along with a piano accordian but I've jammed with concertinas and fiddles quite a lot and don't remember wanting them to back off. Still, if you start singing an unaccompanied song, the way Barry Finn can for instance, you won't be troubled by people playing over your performance.

As for which instruments can play when and how, remember that the recordings you hear are arranged performances. They used specific instruments in specific ways because that created the effect they were trying to capture. The recorded arrangement didn't come about because the musicians were in some musical straight jacket and only alowed themselves to use instruments and voices in certain ways. It was just their personal expression of their music. In a jam session or song circle there is no “arrangement” as such and if you can be relaxed about it, you'll hear some interesting ideas and variations that won't be available on any recording. Think of it as a surprise. Remember that some of the best bluegrass the Stanley Brothers ever recorded used only lead and rhythm guitars behind the brothers’ voices. No one would argue that it wasn't bluegrass.

      - Mark