The second Sunday Folk(ish)Club I'm aiming for tomorrow usually has an MC announcement 'Please don't add instruments unless asked' (Some do ask)

There are some well known songs where it is not unusual for the audience to join in the last line of the verse plus the chorus or even to sing a part of the verse as though it were a chorus e.g. She's o'er the border and awa with Jock O' Hazeldene.

Singing a slightly different version of a well known song does need you to announce that. There are clubs where the audience take a delight in silent attention then joining in correctly.