Well I have encouraged some really awful singers in my time. There is rarely no more than one bad singer in any group. They usually either get better or leave. I have never found it a big problem.

I have also been to groups where almost everyone had a copy of 'Rise Up Singing'. It makes it much easier to sing along when everyone is using the same words. It is not my favorite kind of singalong but it can work pretty well, especially if it a group of newcomers.