There are several "hot spot" controller / routers that have user management and authorization some even with bandwidth control basically configure, plug and run.
If you can find them the older Ornico/Proxim AP 2500's have all this builtin. They are often found on Ebay these days. They are great controllers for what you want. The user opens his browser, sees a login screen, you can have guest accounts, or give them usernames and passwords. You can set the up and down bandwidth per user. Even though it is now discontinued I have several we use for just this purpose and they just work. All wireless clients are isolated from each other and from the wired side of the network.. so you don't have to be concerned too much about your network security.
Another option is like AirEpochs HotSpot unit.
http://www.echotechwireless.com/Airepoch...
It has user management all built in which makes it easy to keep control etc. Just not as many features as the AP2500 but easier to setup.
You can even use the newer linksys with the Boingo system if you want paid service or use some of the older units with Chili Soft controller software which will work with DD-WRT modified units.
Visit http://dd-wrt.com for details.