Sonny's position in the novel is murky. A few hints:

1. He did have some sort of crew. They were disbanded, I believe after the peace conference, and placed in the others' regimes;
2. He is the only one in the Sollozzo meeting with Vito and Tom;
3. Vito was considering him for underboss at the time they moved to Long Beach (late 30s?). As illustrated by the incident with the "furnace inspectors," Vito wasn't happy with Sonny's performance; and,
4. When Vito is shot, Clemenza calls Sonny and says something like "You have to carry the ball, Sonny," which he might not have to say if there was a clear chain of command.