Keep the victim's head facing away from his body, always looking directly forward. This ensures that the head is always face forward keeping that spine nice and straight along the long axis of the body. This is the leader's responsibility. Once the victim is straight up on his side, have that additional 5th rescuer slide the backboard all the way against the victim's body so when he is rolled back down, he will be rolled back onto the board. If you only had 4 rescuers in total, the middle rescuer can slide his hand to the victim's knee and have that last rescuer move around and push the backboard into place.