Much better to tell someone to pack it in, in as friendly and courteous manner as feels fitting, rather than bottle it up and expect "the organiser" to muscle in as the figure of authority.

It's not hard to tell when someone welcomes accompaniment or not, and if you get it wrong only a pillock would get upset about getting hushed. It happens all the time - a raised hand is enough.

What I find awkward is when I'm singing something I've only recently learnt, or only recently made up, and someone playing a melody instrument jumps in and plays what they assume isn't right tune, only maybe it isn't. But there's no problem in just saying "don't join in on this this time round, I'm still running it in!".

And God preserve us from generalised rules like "no joining in" - they take the guts and spontaneity out of a session.