The fact that you are a Broncos fan almost makes me want to not tell you what to look for....but hey football season is over.
Now for what you want to accomplish is no simple task. You are talking about network users on a directory. MS calls their version Active Directory (oftenly abbr as AD). AD servers authenticate against your directory of users and then you create groups with in AD to grant access. A simple example would be accounting, and you give all your accounting users access to financial data that would otherwise be considered private to people out side of accounting.
I suggest you look into the MCSE books as it describes in grave detail what you want to do.
Alternatively you can look into the free open source version called Open Directory (OD). All platforms support the standard open source formats like LDAP, Kerberos, caldav, so on and so forth.