There are two types of session those that a number of musicians turn up to and as Martin says when there is a break in playing you fill it with a tune or set of tunes.There are no rules ,but if someone in the group starts playing you shut up and hear the tune and join in if you know it.There are never long gaps between tunes and nobody dominates the session ,providing there is a reasonable flow of tunes being played.
Martin will remember the wonderful all night sessions at the Balfour Arms Sidmouth.Great times.
The other type of session is similar to that which I used to run at Sidmouth which was a play around and it just required finding out before or during the evening who wants to play and lining them up ,one to play after another. Either method works well with the latter at least everyone who wants to play gets a turn.
Al