Any existing AnkiMobile user that syncs with AnkiWeb is welcome to become a beta tester - please just post a private ticket requesting an invite. If you sync AnkiMobile with a different AnkiWeb ID, please include your AnkiWeb ID in the message as well.
Switching between beta versions and stable versions will preserve any cards you have on your device. But just to be safe, please sync any changes you've made to AnkiWeb before switching versions.
Once you've received an invite, please accept it and install the TestFlight app from the app store. Using the TestFlight app, you can download the latest beta. As new beta versions are released, you will receive a notification, and can upgrade as you please.
If you would like to revert to the store version, you can go to AnkiMobile's app store page and tap on the cloud icon. Once you switch back to the store version, you will receive normal app updates as you did before.
If you no longer wish to beta test and do not want to receive further notifications, you can remove yourself using the TestFlight app.