You shouldn't add them right away, the other chickens in the flock will bully them. Not only that, if they have any underlying illnesses, the stress will let the illnesses come forward. They can spread things to your current flock. You should set up a "see, but no touch" area where there is a shared fence and they can get away if your flock pecks at them. After a few weeks they can range together, and see how things go. They will have to establish a new pecking order, but if they are familiar the flock won't hate the new girl, they will just fight and bicker a little until they work it out. Right now I imagine she is not being allowed to eat or drink as much as she would want because they see her as an enemy. Chickens don't lay eggs under stress. The new change to her home, routine and flock mates will automatically cause her not to lay. She probably got pecked in the eye, if you can post pictures it would help us all help you figure things out. For now you can separate her and make sure she has food, water and vitamins and a place where she is safe and can recover. Post pictures for us.