If you are still working on a draft, you can edit your search and add a traveler. This will run a new search and bring up results with availability for the new number of travelers. Selecting a new flight will not replace your previous selection, so don’t be alarmed when you see two selected flights in the "Review and Book" itinerary page. Simply remove the old selection that does not include all travelers.
If you want to add a traveler to a trip that is already booked or is pending approval, you will need to book that other traveler's flight separately.