Rats are very social animals but it can take up to several weeks to introduce a newcomer. It will take some time and patience. If it is possible, get your rat a litter mate or another rat that your rat lived with before. This will make introductions easier. If not then use the following steps to introduce your rats to each other.
1. Place both rats in separate cages and place the cages near each other. This will allow the rats to scent each other in safety. You don't want to place the new rat directly in with your old one since he will feel defensive and may attack the new guy to protect his territory.
2. After a few days switch their cages so they can get a close up scent of each other. Just for a short while then return them to their own home. Do this several times.
3. Next introduce them in a neutral location like the bathtub, large box …etc. Watch for aggressive behavior such as puffing up the fur, arching their backs swinging their bodies sideways, I've even seen them whack their tails on the ground. If you see signs of aggression then carefully pick up one of the rats to prevent an attack. Use a towel to prevent bites. Then try again later.
4. If they have reached a point where they are accepting each other in the neutral territory then the next step is to let them interact in the area where you play with your first rat. Once again watch carefully.
5. Finally.., Thoroughly clean out the large cage, make sure you clean all the toys, the house, food and water dishes. Then rearrange it so it looks like a new territory. Trim your rats nails to help prevent scratching. Wash your rats with a mild shampoo to give them a nice clean scent that is the same for both of them. Then place them in their clean cage. Its best to do this in the morning when rats are sleepy.
Now be ready there will be some dominance behavior. The dominate rat will "attack" the submissive rat and force him onto his back. This is normal rat behavior, someone has to be the head rat. Watch for the raised hair, arched back, and sideways approach that are signs of aggression. If so, remove one of the rats but if no signs of aggression then leave them together.
Don’t worry if one rat is squeaking it doesn't mean it's getting hurt. This is just a rat's way of saying "I give up". As long as you have followed the introduction process step by step, and as long as neither of the rats are showing signs of aggression, the just let the fight run it's course. If you take the new rat out, you'll prolong this adjustment phase.
If however is it appears that the attack is especially vicious or one of the rats is being injured then you should remove him from the cage. For a few days there may be dominance behavior but before long they will be rat pack mates sleeping eating and grooming each other.