The email address you entered during the signup process is your Between ID.
And to use the app as seamlessly as possible, you have to register with a valid email address, and you have to verify this account in order to use certain features.
[How to verify your account]
- Open the Between welcome email that was sent to you immediately after your signup (only valid for three hours).
- More tab > Profile > Click on the light green banner that says "Please confirm your Between account". (once your account has been verified, this banner will disappear)
[If you're using an ID that can't receive emails]
Please change your Between ID to an email address that you can use to receive emails. Once that is done, you can take the steps to verify your account.