Cats needs to be instroduced to each other gradually, same goes for kids, whose energy can be scary for a cat trying to get used to a new environment.
THis is how you introduce a new cat to other cats:
1. Keep the cats in different rooms for the first week. This will give the new cat time to explore without any threatening behavior and it will give both cats the chance to sniff out each other's presence and begin to familiarize themselves with one another.
2. Let them in the same room for short periods of time (with human supervision). Don't leave them to their own devices; you may need to act as referee.
3. Let the cats smell each other, and look at each other. If they start to hiss, put them back into separate rooms.
4. Every day, let the cats be with each other for a bit longer. Stay with them and let them explore one another bit by bit.
5. Introduce a shared love. Bring in treats! Lay the treats on the floor in the same room. Make sure there are adequate treats for both cats and make two separate piles. The cats will begin to learn that you intend on feeding them both the same good food in the same amounts and that there is no need to feel competitive over food.
6. Share toys and play with both cats. In the same room, try playing with both cats equally. The aim is again to teach both cats not to feel threatened and that they will be treated fairly.
7. Try leaving them alone for a brief period of time. If possible, keep an eye on them from afar.
8. Relax! Let them be. Sometimes they may need to sort out their pecking order alone and unaided by human intervention; once you feel you have done all you can to ease the transition, let go. Finally, the cats will be able to be together in the same room forever.
[edit] Tips
* Let the the new cat explore the house, but don't let it in to the rooms where the other cats usually hang out, because the cats will get jealous.
* Most cats will relax around a new addition once they realize the new cat is no threat to the food supply. Make sure the cats that were there previously always get food, on time and that they never have to compete with the new cat for food. That can ease the transition.
* Introducing a new kitten to a cat may be a little easier but be wary of the kitten's playfulness exhausting an older cat that has little tolerance.
Gradually, very gradually. Stress and cats do not make good mates.
You can read more at the sources I have cited.