If you want to integrate some new hens into your flock, do at least 2. I have integrated many times into my flock, and now have my system down. While it may not work for you, this is what I do:
Place the new birds (2-4) in a separate coop
They spends minimum a few weeks, up to a few months in there, while I "tame" them and get them used to me. It would also be good to examine them and observe for signs of illness, parasites, etc, while in quarantine.
So far, it has been best to let the new birds free range at the same time as the older flock. That way, they can easily escape if an older hen is being overly cruel. Also, just let them get used to the sight of one another. You can even take the youngers into the current coop to get them used to the surroundings when they are fully introduced.
Eventually, if everyone is healthy, carefully put the new hens in with the old, and let them mingle, with supervision. Do this several times, then add the new birds in permanently, preferably at night. It will be best to make sure that there are multiple waterers & feeders, so that you won't have to worry about the new hens not getting fed. While there will be some feathers flying, know that this is completely normal and necessary. If it gets too out-of-hand, you can pull them apart to give the little ones a break. If there is blood, take that pullet out immediately, and treat with Neosporin. Let the wound scab up before you re-introduce her (don't go through the whole process above though). If there is only one other pullet, take her out with the injured one, so that she won't be the sole one getting picked on. While it will be rough the first few weeks, the flock dynamics will readjust and you will have a flock of happy, healthy hens.
Sorry if this was rambling, please let me know if you have any questions!
