If you recall, Emily first remembered that Jenna was driving her somewhere in Season 3, Episode 2: “Blood Is the New Black”. In her flashback, Emily sees Jenna in the driver’s seat. But Jenna turns her head towards the passenger seat with a smile, as though she’s clearly looking at someone next to her. Who could that person be?
One possibility is that Jenna and another member of the “A” team set the entire thing up. That means that someone did the actual digging in the graveyard, and stole Alison’s body, keeping it stored in a big cooler along with some vodka. Jenna could be their accomplice, luring Emily to Ali’s grave with that midnight phone call, waiting to grab her and put her in her car, and dropping her off at the crime scene.
So who would be riding with Jenna? It could be the Black Swan, since we know she and Jenna speak to each other. It could have been Lucas (Brendan Robinson), which would explain why Lucas was walking in Rosewood that night walking to his car. Jenna could have picked him up in town for the ride, and dropped him back off at his car.