Users who've already signed up for the program received the following e-mail:
You can get the OS X Yosemite beta by signing up on the official Beta Program page which can be found here. For those who've already signed up for the program: you can download the beta simply by signing in to your Apple ID (can be done on the same page). A guide covering some basic information about the beta can be found here.Good news! OS X Yosemite Beta is now ready for you to test-drive. Remember, this is beta software, so please carefully review the guidelines before you decide to install this pre‑release version of OS X. To get started, click the link below.
Get OS X Yosemite Beta
To learn more about OS X Yosemite Beta, you can visit the Program Guide, where you will find information about the Beta and how you can provide feedback.
Check out the OS X Beta Program Guide
We look forward to your feedback.
The OS X Team
A few bugs have already been reported by Apple, but I expect at least a few more will be found by committed beta collectors.