buddy on registration

[techu]

how to add a buddy during registration?

I mean there should be a field like "Initial buddy" in the registration form then the user have to input a username(of an already registered user) and when they click submit registration button the new user will appear in the buddy list of the already registered member(the member who`s username was given in  "Initial buddy" input during registration).

please tell me how to do this, I will be very great full..

Thanks.