Sign up by replying to this forum topic. Just saying "hi" will do, and don't write your e-mail address. Once I've registered your account in iTunes TestFlight you will get an e-mail instructions how to download the beta version. The registration step is a manual process, so it can take several hours before you get your mail.
IMPORTANT: I've got many reports of invite emails getting lost. If you haven't received the email within 12 hours, let me know and I'll send a new one.
Sign up for the Android beta here: https://play.google.com/apps/testing/net.openwar.samurai