How do I enabled registration on sites within my network?

I want to allow my clients on my network to have visitors register on their site. Currently, there is no link or button to allow visitors to access the signup page. This looks like default functionality for some bizarre reason. Anyone trying to reach "subdomain.mydomain.com/wp-signup" is redirected to "mydomain.com." The same occurs even if the client is using their own domain name. (subdomain.com/wp-signup)

Instead, visitors should be able to register on the subdomain, and remain there, without ever seeing my domain.

So far in my research, others have recommended Multisite User Management, however this did not do what I needed. I don't want any manual involvement in the user registration process of my clients.

I realize this isn't an issue related to a WPMU product and would greatly appreciate any assistance.

Thanks.