No longer do you need to be a $99 member of the Apple Developer Program to test OS X betas… of course, this isn’t to be confused with testing iOS betas…
What is the Apple Beta Software Program?The Apple Beta Software Program lets you test-drive beta software and provide feedback to help us make it even better.
How do I participate?Sign up today with your Apple ID to access the latest public beta release. Then install the beta software and start using it. When you come across an issue that needs addressing, report it directly to Apple with the built-in Feedback Assistant application.
What else do I need to know about using beta software?Before you install beta software, please be sure to back up your Mac using Time Machine or your iOS device with iTunes. The public beta software is still in development, which means some applications and services may not work as expected. You should not install beta software on production or business-critical systems. We strongly recommend installing on a secondary system or device, or on a secondary partition on your Mac.