You really need to let the chickens get to know each other safely with a barrier so they don’t hurt each other.
Put her right away into a metal cage with some food and water. Then let the chickens out to meet her.
One of our existing flock went up to the cage and they fluffed their necks up at each other.
We put our new hen into the box we brought her home with inside the house. Then everyday we put her out with the chickens in the cage.
There was no more of that fighting stuff but I wanted to get them all used to each other for the week before taking her out of the cage.
After one week I let Dawn out of the cage to be around the other chickens.
Now it’s been 2 weeks and we are able to keep them all together in the coop.