For something like that, you'll want to buy a router which has enterprise options. If you put a router in enterprise mode users will authenticate with an authentication server with their own set of credentials. You'll need to setup an authentication server to make this work, usually a device seperate from the router.
http://community.linksys.com/t5/Wireless-Routers/WPA-Personal-vs-Enterprise/td-p/103306 This explains it better than I just did. As for how to enable enterprise mode, I would look it up on the manufacturers website and google.
As for signal boosting, you have two options. Firstly, you could try using a signal booster. Basically a bigger, more powerful antenna. However, that still runs into the problems normal wifi antennas face i.e, doors and walls reduce signal strength. Depending on the building, that could be fine. The other option is using multiple routers. You can essentially have a bunch of routers spread out over a large area broadcasting for the same network. So, if you have a very large area to cover and plenty of rooms, that's probably the better option. To figure out how to set that up, same thing, manufacturers website and google.
Those who create and rely upon brilliant and complex creations are often destroyed by some idiot plugging an infected usb stick somewhere they shouldn't have.