To try this out, open up Locksmith, and start typing "registration", like so:
Click on the "Customer Registration" result, and Locksmith will create a lock that covers your shop's registration page. From there, feel free to add whatever keys you like!
Tip: Passcode keys work really well for registration page locks. And Remember, keys based on things like customer tags won't work, because only visitors who aren't logged in will see your registration page.