Certbot version 0.8.0 has been released.
The main new feature in this release is the register subcommand which can be used to register an account with the Let’s Encrypt CA. Additionally, you can run certbot register --update-registration to change the e-mail address associated with your registration.
More information about the changes in this release can be found at:
A note to people reading this thread in the future because of questions about registration or accounts: most users do not need to run certbot register at all, because running other certbot commands to obtain a certificate will automatically register an account if you don’t have one.
certbot register should be used by people who have specific needs related to account registration, not by most users.