seems to me that, in a bar, or other open mike, there 'should' be some emcee to do the dirty work when talking gets obtrusive...if the performer has to handle it, it gets more awkward...

In a private setting, or at a camp or casual singaround, it requires various solutions..sometimes just the host saying early on..."...and lets all remember to take conversation and such to the back room" might help...

as far as behavior of participants..(singing off key, or a slightly different tune, or guitarists who 'play along' on everything,) ....sometimes you just have to tell them...even good friends. I know one fine singer who seldom just sits down and joins a sing, but stands on the edge and waits to be begged ...then sings a fine song..and, when finished, usually goes back to gossiping and chatting, I often wonder why someone does not say..."shhh, I'm trying to listen".

And there are other 'clueless' souls, who, though fine singers, seem to feel that grants them the privilege of doing every third song in a group of 10-12 people...*sigh*

Me? oh, I'm perfect!