For me, ONLY if the singer on the stage requests for the audience to sing along on a particular song. If it is a concert where I have paid a lot of money for a ticket I really don't want to hear other people singing in the audience if the performer has not asked for partipation.
It can be extremely distracting to hear the singer on the stage when other people around you start to sing. Usually I have found that very well known folk artists will only ask maybe on one song, if they do at all,(for audience to sing along).Thank goodness!
In many cases the people singing around you seem to have loud or really bad and out of tune voices.
I have never really liked the audience to sing along, period and always hope the singer on the stage doesn't request it.
And no, I never sing along! Not my thing.
So one audience singalong song is ok, but that's it.

I remember years ago going out on a date with this guy and he had the radio on in his car. He said that he liked to sing. So EVERY song that came on the radio, he would sing along with and very loudly. EVERY song! It became very annoying and I thought it was very rude. I didn't complain or say anything but never went out with him again.
He called me but I told him I was busy or something.
So that experience left a negative feeling for me to hear people "singing along" even though I know that is a totally different situation. He was also way out of tune.???? but even if he wasn't, I still would not want to hear someone singing so much in car, especially while driving.