All profiles log in with the same e-mail address and password. This means that your profiles will always log in with the same credentials as you yourself.
When you and your profiles log in to the app, you will automatically see which profiles are available. Choose the one that you want to listen or read with.
If you are in the wrong profile, you can always switch by clicking on "Profile" in the lower right corner and then on "Switch Profile". You can also switch profile by pressing your profile name under your book lists on the "Home" screen.
If your profiles have had their own accounts before joining your family account, please make sure to cancel any remaining subscriptions to avoid extra charges. Family account profiles are not linked to their own accounts or e-mail addresses.
Have you created your account with Apple or Google? If you log in with Apple or Google, you will have to create a password before your profiles can log in. You find the e-mail address that you are logged in with under "Profile" in the app or on your account page under "Account Information". You can order a password reset link to your registered e-mail address here. Use the link to set a password and then your profiles can log in with your e-mail and your new password.