I go with Hilary. Unless you have a really good strong voice it is very difficult to 'jump in' at a mixed session with no rules. You really do have to be ready to start the moment that the music stops.

I've been involved in a number of different singarounds and not really found them stilted. So far as chatting is concerned I go to folk sessions of one kind and another for the music. For me, too much chat spoils the evening, particularly if it carries on while someone is singing.