I was just imagining the kind of impromptu musical collaborations you often get in a jazz setting - but in a folk setting. It's the difference between being a guest who gives a performance, and a guest who invites other performers to join in the performance.

I have seen it in folk clubs. The nearest you get to it is probably when a guest singer encourages the audience to "join in the chorus".

Our local accordion and fiddle club used to do that - at the end of the evening anybody with an instrument (i.e. all the people who'd done floor spots) got up on stage for a medley (described as a "stramash") with the guest band. Worked very well, but the club is in decline and they haven't done it for a few years now. One reason why I don't go any more.