Richard, I agree that folk music is participatory. That does not mean that it is right, or acceptable, to join in with everything, no matter how good a singer you may be. It is about showing respect for the singer, and the song.

Even in a singaround or session, each singer takes his turn and in my opinion it is up to them to say whether the others should join in, not the other way around. Even then, in most cases when someone says "here's one you can join in with" it should usually be taken as an invitation to join in the chorus, not the whole bloody song.

There may be exceptions in some clubs and singarounds, where it may be the custom, but even then one should show some sensitivity towards songs, or singers, for whom it is clearly inappropriate.