wordpress multisite, how to keep user on subdomain throughout registration process?

I am running wp subdomain multisite in combination with domain mapping. This means I can create a new subdomain site xyz.site.com and map the domain name 'xyz.com' to that site.

The problem I have is someone clicks 'register' on the subdomain mapped site 'xyz.com' and they are sent to site.com to register.

This is VERY confusing to many people and I'm getting tired of the complaints.

How do I setup wp multisite, so that if a person wants to register on a subdomain site, that they do not leave that site at any point?

I use multisite to host multiple client sites, all which have their own domain names. I simply 'map' the domain name to their subdomain site. This means that users who come to one clients site to register, are being sent to the primary site to register, which is just plain wrong and confusing