Ellen Polly, I think what we come onto in my terminology is a difference between a singaround/song circle and a session.

In the former, people generaly would take it in turns and probably find it easy to set their own rules for thier performance.

Sessions are in my expericence more commonly instrumental affairs but singing versions do exist. In such a circumstance one could expect a high percentage of chorus songs and an understanding that anyone could join in at any time.

There are of course no hard and fast rules. An example of an instrumental session I used to go to that had the occasional song which was usualy unaccompanied was one in Bangor. The understanding there was that when a singer started, the musicains kept quiet and listened. With a tune of course, the understanding was once a tune was started eveverone was free to join in.

Some sessions may have a leader, others may be complete free for alls, some may have other rules, eg. the Norwich one I attend most is strictly no singing, etc.

The dynamics of these things can be quite complicated. I think as you attend more different ones, you become better at picking up how a particular event works.

Hope that helps a bit.