Two-Car Theory:
Amy (Morgan's roommate at Blacksburg) and Morgan drove up to JMU in Harrisonburg. There, they met up with her old friend, Sarah, and the Guy. Let's say that the Guy drove his own car.
Car #1 (Morgan's Honda) has Sarah, Amy & Morgan; Car #2 has the guy. Or Morgan may have preferred to ride in the Guy's car; in this case, someone else was the third person in Car #1. We can call this person Companion #5. Additionally, we cannot yet rule out other companions in Car #2.
Plus, we only know that Sarah was in Car #1; Amy might have been Car #2. Sarah left later on that evening in Car #1, with one of the companions.
Say the Guy was Morgan's BF. If he were to leave for some reason, she would be motivated to leave early, too. Maybe she had locked her stuff in the trunk of his car, only to come out and find both him and the car missing.
She borrows a cell to call the other girls, and the BF. He says that he will pick her up on the bridge.
This theory is simpler than the Borrowed Car theory, but still has major flaws.
This theory is a good one, If there were more than 5 people going. The need to take the 2nd car would have been either because a) too many people were going to fit into MH's car b) At least one of the people going to the concert had different plans AFTER the concert or c) Someone was late arriving and they were going to MEETUP at JPJ arena.
The need to lie about a 2nd car could be because of possible evidence in the 2nd car??? or to protect someone??