Hello there and welcome to BYC!
You need to separate the new bird in a cage or fenced off area but within the flock at all times. Keep the new bird in this manner for several weeks. Everybody sees, nobody touches. In about 3 weeks, you can let her into the flock. After this long of time, there should be less aggression. Watch them carefully that day and the rest of the week. Keep several water and feed stations out for the new bird and the flock. And always intervene if it turns bloody.
If you keep her separated for several week, she will probably fit in ok. She may always be at the bottom of the pile, but she shouldn't get picked on as she is now.
Good luck and welcome to our flock!