You don't really want to join it thou... The reward is not much better than GNW reward and the competition is way harder. Any group that does not place in top 3 of the X-server will actually get less reward than if they were to opt out. The system as it stands is... badly thought out.

That said, if you really want in, the ranking is based on power (between the winners). You could try to pad it with more players. Most groups have lots of empty space, so if you stop kicking dead ppl etc, you might make it eventually.