Joe's "we ask that they not sing the verses, and only play when requested to do so"
I believe this is how it should be.   I sing unaccomplished and began to sing the Skye Boat song and suddenly three or four people joined in playing the tune - I had to STOP! I find this incredibly rude - after all, do they know my key? my pace? have we practised together? did I ask for accompaniment? NO