I don't think you need to sign up online, I just filled out some form when I got there last year and waited in line for about an hour and got in. If I had wanted to get into the lan or the quake 3 competition I could have gotten in faster as they were pushing those people ahead because not enough signed up for it.
I really should have signed up for the team deathmatch championship and gotten my ass royally kicked, it would have been funny.