You don't have to be in a union to be a SAG member. Usually you have to show proof that you've been in films, that you can act... that sort of thing. And extras can join SAG too, but they have some interesting qualifications for those too. Check out their website www.sag.org
My recomendation though (if your an actor/actress or aspiring actor/actress) is don't call them, let them call you. SAG is litered with wannabes.