Keeping member signup to the single site


I have a Multisite Network set up with lots of (subdirectory) Single Sites.

I’ll call these Single Sites my ‘clients’.

I would like to allow the ‘customers’ of my ‘clients’, to be able to signup and register for the individual website.

However, when I try to register, it keeps taking me back to the main domain of the Multisite.

How do I limit the signup to the individual site only?