Fill in information for first rider and click Register Rider
Paypal "Cart" page will show the 1st rider's information. Click the "Continue Shopping" button in the lower left hand corner
You will then be taken back to the registration page. Enter the 2nd rider's information and click "Register Rider" again
Continue this process (Register Rider, then click "Continue Shopping") until you have registered all riders. Then once you have finished entering all riders click the "Checkout" button on the Paypal "Cart" page.