Here's a serious reply after half a year (only >>5 is half-serious):

> How do you join?

Easy way: get involved in the web scene, hang out on fora and irc channels. Become friends with people; you'll find quite a few are in the scene. Show them your skill, gain their trust and eventually ask to join their group. This method is slow, but if you are genuinely interested it comes naturally.

Hard way: Find a group with a public email address and convince them that you are skilled and trustworthy. This method seems simpler, but it's very hard to convince someone to put their security in the hands of cooldude33@shitmail.com . Do you want to be a member of a group that accepts people as member after a few emails?

> How are groups formed?

There's two parts. First, some guys come together and decide they want to start a group. Second, they get one or more site owners to have them as affil. Again, they must convince them of their skill and trustworthyness. And they must first find them, so creating a group without already being in the scene is uncommon.