Decap and Netlify
Decap uses the Netlify Identity Widget to manage users.
The admin-only solution, says you have to change email templates on Netlify, but that's a paid feature. I'm not going to pay to send one email, so what we'll do instead is send the invite and just tell the recipient to go to <site>.com/admin and see if that works.
Pitfalls
The docs imply you should store admin/ in your site's output folder directly, but this is bad idea. If you ever delete the public folder, admin/ will go with it. Better to pass admin/ to the ouput as a static resource.