It sounds like the question was more for song circle format rather than performers. My default in a song circle is of course if is up to the singer to state her preference but default is sing on verses if you know the song but never overthrow the lead singer and in general never add verses..some exceptions. Never ever add verses from the blue book. Train newcomers to state their preferences as to people singing along and instruments. I love people singing along which is why we call them song circles instead of listening circles. Anyway every place has it's own traditions and state them clearly if you are rigid about this.