Each time you want to sign up for a Handbook account you need to send an email to email@example.com. It would be good to automatically create an account when someone tries to sign in who doesn’t have one yet. That would not only encourage open contributions but also make the signup journey easier.
We currently use WP GLogin for P4 https://wp-glogin.com/glogin/ – and it already has the “Auto-create new WordPress users if they authenticate from your specified G Suite (Google Apps) domains” – but it’s only available in the premium version – that extension would solve the @greenpeace.org access.
We should figure out something similar for all non-staff emails, perhaps installing a powerful anti-spam / reCAPCHA and regularly review inactive accounts