At the top of the registration page, there is text that directs any previous participant to auto-fill the registration form.  The default text reads

"If you are completing this form for yourself and have participated in any Friends Asking Friends in the past, click here to login and autofill this form."

If the participant uses Auto-fill when registering, they will be directed to fill in the username and password that they used in the previous event. If the login information is correct, the registration form will autopopulate with the contact's information and the participant can log into their Headquarters using the previously used login.