I think that last applies more in singing sessions than in tune sessions. I mean, having someone formally in the chair carrying the responsibility for ensuring that things go along well and that newcomers get in. With tune sessins I think it's easier to have a more improvised pattern, in keeping with the music.

Bringing in nnewcomers and keeping things going along are two separate tasks actually, and it can be better to have two people doing them. Preferably in telepathic communication, or what feels like it. But essentially, everyone has a shared responsibility to help newcomers and shy people have a chance.