[Max RNR]

I agree with @Delta for the most part... continual exposure where you are dealing with the same stuff together (like work, a hobby group, or going out skirt-chasing/gaming) is usually all that it takes. Really, it's the same as how you made your kindergarten and gradeschool friends.... you just naturally fall together as friends or enemies -- Bob and I like blocks! I'm gonna play with blocks with Bob! You are not going to play with dolls with Jerold and his fat girl buddy, obviously. Also, high intensity or traumatic experiences where you all have to pull together to get stuff done or survive (military, critical public service works, etc.) also creates bonds and friends. There are also proxy bonds that can become friendship material, such as if you were in the military and are out now go to your VFW or American Legion (or the equivalent if you are not in USA) and talk to your brothers and sisters in arms about the good and not so good old days. Other social scenes that you have to go in cold are much harder to break into... such as you decide to become a yacht "captain", buy a boat, then have to deal with the old boys club at the docks... tho you can work most of that out at the nearby bars if you are cool and now your stuff about boats and always defer to those more experienced than you until you get your sea legs for real.