Prior to the Mac Dev program restructuring at $99 for everyone (back when you got hardware discounts), I'm pretty sure that at least some betas were available to non-developers, but I might be misremembering things.
But I think you had to apply for an invitation, and hope to get lucky and get picked to join.
http://arstechnica.com/apple/2011/04/appleseed-gives-regular...
Good to see they've expanded it to everyone.
It's more or less an 'invite only' program where Apple will tell you (or your employer) is allowed access.
I'm not sure this will give access to 10.10 seeds though - for the last couple of releases we've gotten redemption codes to install them initial seed - it's a separate application, not a software update.