User management Architecture for Web Based Application

This may be an old school question, but i am novice in all web scripting languages, Just Wondering if we can have user management console, in which user initially makes an account over the internet,(so, that i can know, how many users had made account) and then when he logins again second time, that should be intranet Based i.e. i only want him to have one login access to my application.

It may be old question, but i am not getting a clue where to start on that.