Everyone can download It from several sites, but to be able to use It u have to have your UDID regged, = your idevice uniqe ID number must me regged/added on the Beta verification server.
There are here also several ways to get your UDID regged, become an Apple Developer for a small amount of money directly on Apple Developer and get many slots that u can reg OR pay an even smaller amount of money to have some Dev add your UDID for you ex: Activate My UDID ? Purchase with Google checkout.
And once your UDID Is regged u are able to install even the future betas.

