First, have that person fill out an RSVP card with all of the information you will need, including credit card number.
Next, enter the person's information into the system as if you were that person, filling out the online form. You should have all of the information you need, with the exception of the user name and password. Use the convention of FirstnameLastname as the username and LastnameFirstname as the password. The user will be able to change the password at any time in the future.