Puppy bites!

I'm asking quite a few questions here on DG but I really love the very helpful responses I get from you guys!
I found a pitbull puppy about 3 month ago that was abused and underdeveloped. She is about 6 month old and doing much better now. It was very hard to get her trust and make her feel save but even my friends can come close to her now. But whenever I take her to the beach or the park and let her off leash with my other dogs she starts running after people and tries to bite in their ankles. She also barks at almost everyone like they're going to come over and hit her. She feels so threatened! When I take her for a walk she tries to hide under cars or wants to turn around as soon as a person comes along. What can I do to make her feel save??? I don't know what the poor thing had to go through but I can only imagine...