How do I get Network Users to Site Users automatically?

I’ve set up three niche network sites and am preparing to release affiliate membership accounts, which I anticipate will result in quite a few members.

The problem I have thus far is that when a user registers, it appears in the Network Users and not Site Users. To get Network Users to the Site, I need to manually add them through the Site Admin Users menu.

Is there a way that users who appear in Network Users can be automatically pushed through to the Site Users section?

Thank you,
Buck
San Francisco, CA