You can enroll but you don't take a class until after you graduate. We were informed that the 3Ls who locked in their Barbri rate in their 1L year payed $300 less than the 3Ls who were just signing up. You also get some free stuff like a study book and a Nalgene. The PMBR, I can't remember what the savings was supposed to be but they also give you free stuff like some study guides.
It's really up to you. If you are sure that you are going to use Barbri etc.. and you have the money, (it was 50 for PMBR, I can't remember what it was for Barbri)then go ahead and pay, get your free loot, and save yourself some money in 3 years. We had until October or November (can't remember) to decide if we wanted to put a down payment. The reps had tables set up in the meantime. If you still have questions, ask the reps at your school, they're there to help you. Congratulations on your acceptance and good luck with school.