Parent/Child Registration on Multi-Site

Hello,

Can someone point me in the right direction for some material on setting up Multisite registration? This is what I'm trying to achieve:

Parent Site: Main hub for my network where users of child sites can interact on a large-scale level

Child Site: "Micro-Network" where users register and must be tagged/registered as being from this specific child site.

The theory: This is a project for several condominium buildings in a community where the building managers will operate their own child site and residents must register through the child site. The parent site exists to tie them together and offer communal Buy & Sell and Q&A style pages.

I'm new to multisite, please forgive my inexperience.