How can I set username and password for my wireless users for connecting to the Internet?

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I have a wireless network which everyone can connect to use the organization's portal. But I want to restrict users from connecting to the Internet. I have seen some organization do this by setting username and passwords for legal users. e.g. when users type a website address, another page opens and asks for username and password. How can I do this? Is it a DSL modem configuration?
( I use a TP-Link wireless ADSL router to manage users and connect the organization to the outside.)