Buscetta enters the story out of nowhere. But, he would have had to have had some connection with the Corleones to turn up on such short notice (the train scene). Frankie spotting him and saying, "We got company" affirms that he was a known commodity.
I've always assumed that Buscetta is one of Don Tommasino's men, Michael drawing support from the one well that he is sure he can trust. In the scene where Vito kills Don Ciccio, Don Tommasino is shot, and the driver and another man, seemingly useless to the plan, get out of the car and help carry Tommasino to safety. I've wondered if that fourth man isn't young Buscetta, although it could just be that Coppola thought that carrying Tommasino looked cooler than dragging him, and that required an extra man.
