Only one sign in option ips sites

[kmk]

Hello, I recently try achieve this

I would like all sites members when to click the sign in, they are only can see the server site button to join, if an guest decides sign up, then will use master sign up form to register.

My goal is only offer the master site to sign in or sign up. 

This is an new idea for request the feature? 

Or already there any option that let me do this? 

[bfarber]

You can redirect all registration requests to an external site via the existing AdminCP settings already.