OS X Beta seed program is now open to anyone

Apple has opened its OS X Beta seed program to anyone with an Apple ID, via the new OS X Beta Seed program. Simply sign in at Apple's beta site, agree to the terms and conditions (including a prohibition on sharing information about the beta versions), and activate the Mac you want to use with the provided Beta Access Utility. Once you're live, the beta builds of OS X will appear in the Mac App Store.

More info here:
https://appleseed.apple.com/sp/betaprogram/welcome
and make sure you have a solid backup before testing