You can't. They take a certain number of beta testers and that's it.
And were you really prepared to run an app that might not work, and report every problem to the people running the beta test? (Beta doesn't mean "I get the app before anyone else does:, it means "I'm going to be running an app that might brick my phone, that I have to bang on almost as if it was a paid 9-5 job, and report all problems immediately as soon as verify them".)
Is that really what you want? If not, PLEASE DO NOT JOIN A BETA TEST GROUP! Too many of them have "I got it before anyone else" types - and the app gets released with tons of bugs that were never reported.