Have Users enter a current username to register

I’m currently creating a site that only allow a user to register for the website only if they input the username of a current member. If the new user do not input the correct username. They will not be able to sign up… The section a user will have to enter a current member username would be called sponsor name.

I will like signup page to look let this

Username (require) :___________
Sponsor Username (require):__________
Email Address (require) _____________
Choose a Password (require)___________
Confirm a Password (require)__________