Generally, you cannot simply "add" a person to an existing airline confirmation number once it has been ticketed. Most airline systems are built to lock a reservation to the specific names entered at purchase. If you want a friend or family member to join you, the standard procedure is to book a completely separate ticket for them. After booking, you can call the airline to "link" or "cross-reference" the two separate confirmation numbers; this tells the airline's system that you are traveling together, which helps if there are delays or rebookings. If the person you are adding is an infant (under age 2) traveling on your lap, most airlines do allow you to add them to your existing reservation via their "Manage Booking" tool or over the phone for a small fee or tax. For adults, if you want them on the exact same itinerary, you would typically have to cancel your original booking (subject to fees) and rebook a new one for both people together.