'Choruses' are for joining in with - that's why they're called 'choruses'. Verses are not, although there do seem to be a very few songs which have become communally held - 'Adieu Sweet Lovely Nancy', 'Spencer the Rover' come immediately to mind, and maybe the odd first verse of some songs. But, by and large, I would consider it rude to join in with verses, and it seems to me to be a sure way of 'standardising' songs, rather than letting them evolve differently with the different people singing them. That would be sad.

Barbara