Self-registration / Request to Join

On the login screen of a Ryver site, there needs to be a way to self-register (with approval from an admin, of course), or a way to request to join, which an admin can then approve.

This way, I can send a link to the site to the users in my organization and they can sign up without me having to invite them individually.

(It is better if they request to join, rather than me "forcing" the solution on them).