Like all these terms "singaround" can mean anything from a truely spontaneous sing when people have got together for another reason through to something pretty close to an open mic session. The amount of organisation required varies accordingly.

Non-performers is an interesting question. If you don't want non performers you don't sing in a public place or advertise the event to the public. One thing that pisses me off is what are clearly intended to be "performers only" singarounds advertised to the public as folk clubs.