When you are trying to add travelers to an itinerary, please follow these steps:
Step 1: select the itinerary or itineraries you are booking
Step 2: you will be redirected to the trip summary page. You can change the number of passengers by clicking on the "Add traveler section" (this is located on the top right-hand corner).
Step 3: after you select "Book," you will be taken to the check-out page where you can enter passengers' names and your billing information to complete the reservation.
Please, make sure that all travelers that are being added to your itinerary have government issued photo ID on the day of the trip.