Apple has started sending out OS X Yosemite builds for Beta Program, as announced yesterday. The build is being sent out to a million Beta Program members. Participants can get a redemption code from the site, to redeem in the Mac App Store.
Apple has said that Apple will not update the Beta Program Yosemite builds as often as the developer seeds (which come out every two weeks, usually), but participants will be able to upgrade to the final public version of Yosemite seamlessly through the App Store, when it is released. The seed released in the beta program is one build newer than the one released to developers on Monday.