I've definitely come across this problem, and I react differently, depending on how bad it is!
I've shaken my head slightly at some, ignored others, nodded at others to encourage them to join in fully (most 'round here join in softly, at first, to see if they're 'welcome').
Still other times, I've pushed bravely onward, wishing afterward that I'd had the nerve to speak up! (The worst was at an open stage, where a guy decided that Sally Rogers' "Lovely Agnes" needed an "oom-pah-pah" backup! While it *is* in 3/4 time, it's not a straight 3/4--there are pauses between vs. & chorus, etc.--places to breathe. These were all neatly obliterated by Mr. Oom-pah-pah--and I finished up gasping for air! That was an early-on experience; nowadays I would find a way to deal with it, and not 'endured to the end.')
From all the replies, I'd say there's not just one way to deal with this problem!