Introduction should be made slowly and gradually. That means the newest cats should be in one room, that way, the cats can smell each other from under the door. Also take a small baby blanket or piece of clothing, rub it on the cats and then let the other cat smell it and so on. After that, you slowly take them out for short periods of time but do not leave them alone. After a while, they should be ok but you never know with cats, some never get along.....
As for me, I have always just plopped the cat carrier in the living room. the cats smell it and then I let the cat out. The others would follow the newest member....
Good luck!