There have been so many threads on this topic. Here is a farily recent one.

http://www.flyertalk.com/forum/showt...2&page=1&pp=15

Basically you get invited to join. The criteria aren't published and even we don't know what gave us the edge over some others.

The major perks are - being above 1K in priority for most things (which is very nice), having worldwide toll-free access numbers to the GS agents, and the feeling of how important you are .