You need to gradually introduce them. Put the new kitties in a room. Rub them down with a towel and let your other kitties sniff it. Do the same with your kitties. Let them meet eatch other under the door. Eventually, after a couple of weeks, let them out to explore. Keep a spray bottle handy with water in it in case there are any bad confrontation, i.e., biting, fighting, etc. It's okay for hissing and growling, as the older kitties are trying to let the new kitties know they were there first.

It takes time and patience to introduce cats. I hope you will take the time to help all cats involved to get to know one another.