ANDROID: You will shortly receive an invite to the alpha testing via Google Play. You can then accept the invitation and download it via the Google Play Store.
iOS: You will shortly receive an invitation to download the game through the TestFlight App. Feel free to download the TestFlight app from the App Store here: https://itunes.apple.com/us/app/testflight/id899247664?mt=8
NOTE: The game is nearing public beta, yet is still incomplete. It is subject to crashes and major changes. Save data may be overwritten at any time.