mg and Falcon, yes, there are some people who sing so badly (notice I didn't say "who can't sing") -- either by virtue of being off key or by having voices that grate -- that they can ruin even a group sing. Especially if their voices are also loud.

(I have a friend who was once told - quite astutely - by a voice coach, "You have a great voice but you don't know how to use it." This man could be a wonderful singer if he would drop some affectations he's picked up, trying to sound like some other singers, and if he didn't sing so loud that he drowns out most of the others. But he does use those affectations and does overpower other voices, and it has sometimes messed up a good group sing.)

But it's a matter of degree. A lot of people are told "You can't sing" just because their voices sound more like Bob Dylan or Nancy Sinatra than Gordon Bok or Judy Collins. There are many people whom few people would ever care to hear in concert or pay to hear at a pub but who nevertheless can be good participants in singarounds or other informal gatherings when they sing a song or two.

There's a vast range between "makes my ears bleed" and "isn't ready for Carnegie Hall."