Can you add a passenger to an existing flight reservation?

No. No airline lets you add a person to a ticket that is already issued. A booking is tied to the passengers named on it. What you actually do is book a second, separate ticket on the same flight, then ask the airline to link the two reservations so their systems know you are travelling together.

Why airlines don't allow it

A ticket is a contract between the airline and the specific people named on it. The names, the fare rules and the price were all fixed at the moment payment went through. Seats are also sold out of fare buckets — small pools of inventory at a given price — and the bucket your friend bought from may have closed the same afternoon they booked.

Adding a name would mean voiding and reissuing the ticket at today's fare for everyone on the record, which is exactly the thing airlines are least willing to do. So the reservation systems simply do not offer it: there is no "add traveller" button on any major carrier's manage-booking page, and phone agents cannot do it either. The only names an agent can touch on an issued ticket are spelling corrections to an existing passenger.

What to do instead, step by step

1. Get the flight number and date off their confirmation. You need the two-letter airline code plus one to four digits (DL 264, UA 918), the departure date and the two airport codes. Ignore their name, their six-character record locator and their ticket number — you cannot use them and should not be passing them around.
2. Book that exact flight separately. Search the same route and date, filter to that airline, match the departure time, then confirm the flight number before paying. If you'd rather skip the searching, you can drop their confirmation screenshot into SameFly and it will take you straight to that flight.
3. Call the airline and ask them to link or cross-reference the two record locators. Have both six-character codes ready. It is free, it takes a couple of minutes, and most carriers still cannot do it online. See how to link two separate reservations for the exact wording per airline.
4. Choose seats after both tickets are ticketed. Open the seat map on each booking and pick adjacent seats. Do this in one sitting, on both records, before someone else takes the pair.

You will probably pay a different price

This is the part that surprises people. Fares move constantly, sometimes several times a day, and the cheapest buckets sell first. If your friend booked in March and you are booking in July, you may pay double for a seat three rows behind them. That is not an error and it is not something an agent can override.

Being on the same flight has nothing to do with paying the same fare. Two passengers in adjacent seats routinely paid wildly different amounts. Nor does booking together protect you: when two people are booked on one record and only one cheap seat remains, the airline prices both at the higher fare. Separate tickets are often the cheaper outcome, not the worse one.

Does linking guarantee seats together?

No. Linking means the airline knows you are travelling together. That matters more than people assume — if the flight is cancelled or the schedule changes, a linked party is usually rebooked onto the same replacement flight rather than split across two, and gate agents can see the association when they are shuffling people around.

Adjacent seats still have to be selected. On basic economy fares they often cannot be selected at all: seat assignment happens at check-in or at the gate, and the algorithm has no obligation to keep you together. If sitting together genuinely matters, buy at least the next fare up on both tickets. More detail in sitting together when you booked separately.

Adding an infant is the exception

There is one real exception, and it is the case most people asking this question actually have: a lap infant — a child under two travelling on an adult's lap rather than in their own seat — can usually be added to an existing booking.

An infant does not occupy a seat, so no fare bucket has to be reopened. On domestic US flights a lap infant typically travels free; on international itineraries there is normally a fee of roughly 10% of the adult fare plus taxes. You add them by calling the airline with the reservation code and the child's full name and date of birth. Some carriers (American, United) let you request it through manage-booking and confirm by phone. You will need to show proof of age at check-in, and there is one lap infant per adult.

A child aged two or over needs their own seat, and that means a separate booking, exactly like any other passenger.

Airline by airline

American Airlines. No passenger can be added to an issued record. Book separately, then call Reservations and ask them to cross-reference the two record locators. Lap infants can be added by phone.

Delta. Same rule. Delta agents will "link" or "associate" the two confirmation numbers when you call; ask them to note both records. Basic Economy tickets still block advance seat selection even once linked.

United. No additions to a ticketed reservation. Call and ask for the two confirmation numbers to be cross-referenced. United will also add a lap infant to an existing booking by phone.

Southwest. Each passenger is its own confirmation number. Southwest now assigns seats, and travelling parties who ask to be linked by phone are handled together; changes are free, so if the seats are wrong you have more room to fix it than elsewhere.

JetBlue. Book the second ticket separately, then call and ask them to link the records. Blue Basic restricts advance seat selection, so pick a higher fare if adjacency matters.

Alaska. Same again — separate booking, then a phone call to associate the two confirmation codes. Alaska agents are generally happy to move seats around at the same time.

Across all six the pattern is identical: no additions, a separate ticket, a free phone call to cross-reference. If you want the full walkthrough of matching their flight in the first place, read how to book the same flight as someone else.