There are a lot of reasons as to why you need to introduce or re-introduce rats to other rats. Make sure before you try and do any introductions that your rats are healthy, if they are new you may want to consider keeping them on their own for 2 weeks to make sure they aren't carrying anything that may harm or cause sickness to your other rats.

Introducing rats can sometimes be difficult, but if you go about it the right way you should have an easier time doing so. In quiet a few cases it's not as easy as just putting the new rat in with the old rats, this can be a good way of getting your new rat hurt or worse.

I find the best way to go about introductions is to start by putting the new rat in a cage next to the other rats cage or if your cage is big enough put the small cage with the new rat/s inside the bigger cage (make sure there is no way for them to get in or out) leave them like this for a few days. Then during play time put all the rats (new and old) in a play area that is neutral, let them run around and smell each other. Expect some wrestling and "rough" play, when I say "rough'" I don't mean drawing blood by biting each other, if this is happening separate them straight away!!!

If they are just jumping on each other and power grooming each other, a bit of squeaking is expected, this will usually continue till the alfa of the group has pinned the new rat down and groomed them, if the new rat keeps getting up then the alfa will keep doing it till the new rat gets the point. You should let them play like this for a few days before letting them share the one cage. Before putting them in the same cage I suggest giving the cage a very good clean to remove any territorial markings. If you continue to have issues with introducing try giving them all a bath with some baby/small animal soap and then rub some vanilla essence on their back so they all smell the same, then try putting them in a neutral area again to play, you may need to do this a few times before they get along.

 But keep in mind like us some rats just won't get along no matter what you try, so you may need to keep them in separate cage permanently.