We had roughly 10k beta testers, many of which found eachother (we have team features and you can create accounts/invites for other people in your team), we use

We had roughly 10k beta testers, many of which found eachother (we have team features and you can create accounts/invites for other people in your team), we used beta keys, invite codes, all kind of things. Twitter and blogs also played a big role. Big ones (mashable and such) gave us spikes in new signups, but other blogs (lifehacking.nl) and other more targeted sites gave us higher quality subscribers. We haven't done any adwords at all, even though I have a bunch of free $50 adwords vouchers. It just didn't seem worth it for a private beta. We plan to talk about this in more detail in a future blog post, when we're fairly certain that our approach was successful.