I agree that it's best for the organiser to ask everyone attending a singaround if they want a turn, but it's also a good idea if you want to have a go, to speak to the organiser first to make him/her aware.

I went along to a club session first as a non-performer, partly because I wanted to see what standard the performers were, and how it went, etc. I found the standard varied from excellent to terrible, so I felt OK as I just fitted into that range ;-)

If everyone had been brilliant, I might have thought twice before pushing myself forward.