Well if by "register" you mean join leagues to compete with other Quake 3 teams, its really dependant on the area of the world you live in because of ping/latency issues (I hope that wasn't patronising). What continent do you live in? I, fore instance, play Counter-Strike 1.6 and live in Europe, using leagues such as: http://enemydown.co.uk and http://www.thesgl.com/.

If you just want some practice games, get mIRC. I'm sure you'll be able to find people who want to play a match against you guys in Quake 3.