On the registration page, each user has the option of inputting the username of the user who referred them to the board (if he or she was referred to the board by another user).
The referrer can only be set during registration, in one of two ways:
The referrer's username can be directly inputted to the referrer textbox on registration.
Each existing user has a unique URL which they can give to non-members to register with. This URL inserts the existing user's username as the referrer when the guest registers.
The URL is in this format:
Replace [uid] with the referrer's user ID.
Replace [username] with the referrer's username. (In MyBB versions > 1.2.0)
Please note, there was a bug in the referrer URL feature in MyBB versions < 1.2.0.
You can view the users that a specific user has referred by searching for the user in the Admin CP, and selecting "Show Referred Users" in the menu.