DON"T introduce her to the flock, THEY ALL know she is a stranger.
Instead, introduce a middle of the flock bird to her, in her territory. There will be a dust up, but one on one is not bad. Let that settle a couple of days.
Then depending on how many you have, add another pair to the new pair, in the old girls territory. Again, might be a dust up, but should settle quickly. Mostly bluster is what I would expect. Wait 5 days.
Now you should have 4 birds, getting along pretty good. Depending on how worried you are, you can do it two ways.
Free range both groups together, let them go back where they slept the night before. If there is a wicked bird, who is merciless - lock her up. But chasing 4 birds is exhausting, and they usually quit. Eventually you can close up the coop you don't want used in a few days, and they should roost together.
The other option, is a bit of monkeying around, but it does really help with territory. Chickens defend territory. And Chickens know when they are in strange territory, and usually are pretty confident they don't belong, and seem to bring on the harassment.
To prevent this, at night, flip coops. After they have gone to roost, chickens are pretty easy to handle. So put them in the opposite coop. They wake up in a strange place, but because they are with their friends, and there are no other birds, they can explore without being chased for their lives. This lets them explore, figure out hideouts, find feed stations, and explore the coop in the daylight. Leave them there a couple of days 2-3.
Then let them all out together late in the afternoon, and lock up the coop you don't want them using, and they should all go to roost where you want.
Mrs K