Is there a public wiki where this is installed? I'm just curious how the UX works.
Topic on Extension talk:InviteSignup
There is one not too pretty special page, which is restricted to admins. The users get emails with link which allow them to sign up using the normal signup form.
I think it will be nice to have this screenshot within the info box for this extension.
It is always possible to check if a public wiki uses an extension by clicking the "Check usage and version matrix" which you will find a the bottom of the info box.