So.. I just tested this on my instance, and it's possible to directly insert a user entry into the database..
With the right permissions, you can manually add an account that can be shared, and the e-mail address doesn't need to exist, but it needs to parse as a valid address. It's definitely a hack, but it might get you going for now?
(edit: I copied
password_salt from an existing account)
(edit2: directly inserting the user/email bypasses the email confirmation step)