I have a question about ‘For He’s a Jolly Good Fellow’

When you’re singing “For He’s a Jolly Good Fellow” in a crowd, how do you know whether everyone is going to follow up with “Which nobody can deny” or “And so say all of us”? Do people decide ahead of time? Do you just go with whatever version the loudest person is using? Is it bad form to beat up anybody who’s not siding with the majority?

Asking for a friend who’s much older and owns a lot of wool and boating accessories.