I find there are often at least 2 types of sessions. The most common is the free-for-all where there is always plenty of opportunity to start a tune and the accepted norm is everybody who can joins in. These are fun and very welcoming for both beginners and experienced. Singers are also usually welcomed as long as they don't dominate.

The second type is usually set up by a group of professional musicians and they want to play their music in their style and at their pace whilst tolerating others joining in who know the tune. Here you should wait to be invited before leading off a tune. This IS their prerogotive as they set up the session. If you don't like what you see you go elsewhere or set up your own session.

I'm happy with both of these. I understand the second option isn't very encouraging for beginners but there are other options.