All memberships are tied to a phone number, when you first log in using a new phone number you will automatically create a new membership.
If you're already registered as a member you will automatically log into your old membership when you enter your phone number on the login screen.
To register a membership in the app you need to write your phone number according to the format +46 or 46. You will then receive a text message with your verification code, once you've entered this code you will be able to log into the app.
Your phone needs to be connected to internet when you register for a membership. Also make sure your phone's language is set to English, Swedish, Norwegian, Danish, Finnish or German and that you have automatic time settings activated.