This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
you get invited by an existing member (who can invite someone else for every X emails they send I think). Google's way to give their service a patina of exclusivity (which I guess increases the resale value of their advertising).