https://appleseed.apple.com/sp/betaprogram
Join the OS X Beta Seed Program and accept the Beta Seed and Confidentiality Agreement. Apple will provide a Beta Access Utility for your Mac, which gives you access to pre-release versions of OS X in the Mac App Store Updates panel.
If you want to get involved, you will need to go here, after which, you may enter your Apple ID credentials.
Once you’re signed up, any pre-release beta builds or subsequent updates will be seen through the Mac App Store, and with Apple supposedly planning to revamp its desktop software to offer a more iOS 7-like aesthetic, it’s likely that quite a few Mac users will take Apple up on this new offer.
Join the OS X Beta Seed Program and accept the Beta Seed and Confidentiality Agreement. Apple will provide a Beta Access Utility for your Mac, which gives you access to pre-release versions of OS X in the Mac App Store Updates panel.