Very commendable and I have done similar at home before (though I didn't require people to login)
The easy way to do it (though doesn't quite satisfy all your points) if to use the BTfon service, for a small price you can buy the btfon router which gives others the ability to use your connection and also enable you to use any other BTfon acess point in the country (there are loads about!)
http://www.btfon.com/...end?partner=btfonjoin
The harder way would be to setup a dedicated gateway to manage the service, this would require a high level of knowledge on your part and isn't really something that can easily be covered on here.