If you click through to register, the following are listed as features:
* Notification of API changes
* Early access eligibility on select features
* Eligibility for development licenses for GitHub products
* GitHub profile membership badge
It sounds like it just puts your user into a special feature-flag bucket and gives you a badge, for now.
It's the rightmost item on the landing page - you can contact GitHub staff and receive access to develop against their enterprise/standalone product (presumably a license to install a local instance for testing, although I guess they could have a test instance set up for you).
