I think Marje nailed it. And Terry when he said that if someone is leading something, others follow. I'll often put my mandolin down if there is another mandolin player accompanying a song, because I feel that what I would do would not fall to hand quite well with other mandolin players who just may happen to feel an accompaniment differently from me.

And speaking of listening - - -

There is one person who has posted on this thread who I have seen join in on an instrument with an unaccompanied singer and force a key change as a result.   

I've seen another poster here talk right over someone trying (7 times) to start a song - and then criticise the singer because the talker was "performing" a joke.

I've seen a banjo player (not represented on this thread) play right over everyone (yes, everyone) who tried to start a song - when he was a visitor to the singers' session (and not I add a booked guest).

No names, no pack drill.

Incidentally, I think of "noodling" as something quite different - a tendency of some guitarists or instrument players to play something, anything, when there is a silence - and it may or may not be an attempt to start a song or tune, or a bit of private practice, or just a compulsion to finger the strings, and one does not know which. I don't call a purposeful (even if mistaken) attempt to accompany "noodling".

I did have one embarrassing experience arising from this - a person somewhat given to proclaiming their instant expertise on any instrument they happened to buy was - I thought - "noodling aimlessly" - so I carried on talking, only to find later that I had talked right through a set piece they were playing to demonstrate their expertise on their new instrument!

On the other hand there are some songs where surely everyone expects silly versions to be sung or silly sound effects to be inserted - perhaps the greatest example being "Pleasant and delightful" - even down to the added verse to the words of "Pinball Wizard" (or the added verse to "A sailor ain't a sailor any more".