How do you join World of Warcraft cataclysm beta

You do not join per se, you are invited by Blizzard. To make yourself eligible for Beta invites, go into your Battle.net; there should be an option for "Opting in" to Beta invites for their games. Simply select them and download the little tool that will record your PC's technical specifications for them.

Then all you can do is wait and see if you get lucky.