We just introduced third adult boy to 2 others. First week he was in isolation and others got to get used to him being behind a door (he made his presence known by wailing for company). We fed at his door and exchanged stuff the during that week, then started visits to each others quarters (this was hard for one of the brothers, he got very worried of the new guy having been to his feeding and toilet area).
Mid second week we did not see the need to isolate the new guy for nights anymore because despite some stiffness on the part of one of the brothers, a peace had been established. New kitty would not let himself be groomed by (eager) others until the third week, and still only allows it on the head/face. By fourth week the new guy was snuggling and wrestling with everybody.
The whole thing was hardest on the most insecure of the (resident) cats. The newcomer made it very easy being a total gentleman and accepting some growling, hogging of his cup and invasions of his safe space with calm "I am just glad I have company" - attitude.