[Andrew]

We have had three since 2000 and are now fostering a fourth, who will stay with us until he gets a permanent home. We kept #4 (Milo) shut in the guest room (with food, water & cat box, of course) for 48 hours and then let #1 (Watson) and #3 (Linus) in to investigate. #2 (Rita) is generally shy and wouldn't go in. Milo hissed and growled and the other two found this very confusing -- they just sat and watched him, kind of looking at each other as if saying "what is this guy's problem?". There has been no violence, though, and Milo is slowly becoming less of a butthead -- he doesn't growl at the others every time he sees them, just sometimes. In his defense, he'd been an only-pet for most of his 2-3 years, and his only experience with other cats was with a domineering alpha female.

Anyway, I'd start the new addition in a room with the door closed for two or even three days. They'll sniff each other under the door. When you're ready to open the door, put the new guy in a carrier and let the other(s) in to investigate. Let that play out for a while, until everyone has calmed down a bit. Then you should be able to open the carrier and let them mingle.

Good luck and let us know what happens!