I think Jim's list is a reasonable one. What he suggests does seem to represent a reasonable minimum standard for floor singers on a guest night.

However on a singers or singers & musicians night, I think that this can be relaxed somewhat. Folkworks tend to refer to their end of term or end of workshop concerts as "sharing" concerts and to me that is what a singers night is much more about. About sharing songs with others. It is reasonable in that context to try things out. It is not the end of the world if some singers are using words. I know people who feel the need to have the words in front of them who still give a creditable performance.

I do agree about not joining in on non chorus songs - or even verses of chorus songs. It can be most offputting. I recently was completely thrown by this. It was a song I knew well, but was less secure with the accompaniment which I had been working on. Folk joining in on the verse threw me and I messed up the accompaniment. I was not happy about this, as much with myself as with any, but I think if people had just joined in the chorus that would have helped greatly. I have known someone stop, ask people not to join in, then start again. Maybe more should bite the bullet and follow that example.