To sign up enter the username and password form and enter your e-mail as well, leave 2FA empty. You need to confirm your account afterwards in your mailbox.
It’s not completely finished yet, but main functions work fine. It’s a pre-release. (v0.1)
The settings panel needs some work, so it’s better to leave that alone right now.
It’s source is available on GitHub: https://github.com/bschilperoord/OpenBook