It sounds like what you need is a domain. A domain is a group of computers connected to one or more domain controllers. When PC's are in a domain, you then have the option to do the things you wish to do.
To do this, you first need a domain controller. A domain controller is a server dedicated to the function of the domain (i.e. Active Directory, Sites and Services, etc). The catch here is you need a Windows Server operating system and most likely a CAL license, depending on how else you choose to use it. So what I would do first is price things out for a physical server. If you're only using it for a DC and that's it, it does not need to be beefy. However, if you're also going to use it for a file server, it will need some horsepower.
Once you have the server and the operating system is installed, all you have to do is setup the domain and join the PC's to do it and you're ready to go.
I know all of this seems overwhelming, but if you need more help, just let me know.