There are grey areas re above post, The harmoniser might be brilliant[it does happen].
it is courtesy not to join in, unless you know the person well and know they do not mind, this applies to percussionists in instrumental sessions too, it is not a free for all, however this means you run the risk of losing the occasional brilliant percussionist/guitarist, the general etiquette is do not join in unless you do it very quietly, this should apply as regards singing too, unless the singer encourages you in the chorus, likewise chorus singers should listen to the lead singer and follow, not impose or slow down in the manner of Les Paisley[ a lovely man with a fine voice ,but one who was guilty of this 25 years ago , so it is not anew phenomenon
enjoyment is the name of the game but one persons enjoyment should not ruin another persons enjoyment