Joe,
If I might.....I can only comment from the point of view of this side of the pond; whereas unwanted accompaniment can be a damned nuisance, adding bits on to someone else's song, particularly changing the tenor of the performance is even more bad manners and annoying. Is it really okay to do that in the States? I've seen people get away with it in a free-for-all barroom situation, but In a singaround, never!