No, Pedropete is correct, she could have left anytime. All it would have taken is to hop over the fence and call for help. But that is a lot easier said than done when you have been in that situation for a long time, you become sort of paralysed by the fear of the unknown. And I don't think it would have been so much a question of Garrido letting the girls go with her, rather a question of whether they would have wanted to go with her (they would have thought she had gone crazy).
You can't really second guess someone who has been in that situation, I mean who can honestly say they haven't encountered something in their life when they should have done or said something, but didn't, and afterwards couldn't understand why they didn't. The position Jaycee was in was really just an extreme version of that, and the recrimination she will feel towards herself once she understands how simple it would have been to stop it all, sadly, will be just as extreme. There are a lot of difficult things she is going to have to come to terms with, the hindsight thing is going to be a big one, another I think is going to be the paradox presented by her girls.