What happens next?
iOS / macOS: You will receive an invite from Apple's TestFlight program. Follow the instructions to install the TestFlight app from the App Store - Notion will be available from within the app. TestFlight can notify you when updates are available. Note if you selected both iOS and macOS, you will receive two invites (and need to accept both)
Google Play: You will receive an email from PreSonus advising when you have been added to the program. You then need to search for 'notion music' in the Google Play store. This will automatically be the beta version. Note that you do not get notified for any beta updates by Google Play store - you will need to check in the store.