If I have a big group travelling together, can I book them all in one go through the flight API?

Not in one single booking once the group gets past the size an airline allows. Flight booking APIs follow the same rules the airlines themselves use, and most airline reservation systems only let a limited number of passengers sit on one ticket record. Up to that limit you can add every traveller in the same search and the same booking, and they will share one reference, one payment and one set of tickets.

Above that limit the API will normally ask you to split the travellers across more than one booking. That works fine, and everyone still flies together as long as the fare still has seats left, but each booking gets its own reference. If you need the whole party tied to one reference, or you want the special fare and the longer payment deadline that airlines give to genuine groups, that is a group desk request at the airline and not something a normal booking API handles. Some suppliers offer a separate group service and some do not, so this depends on which suppliers are connected to your portal and on your plan.

One thing to watch is that the price you see in a search is only held for the seats still selling at that level. A large party can use up the cheaper seats partway through, so the last few travellers may come out costlier than the first few, and the total you quoted can move before payment goes through.

Before you promise anything to a group client, ask us which of your connected suppliers allows larger bookings and whether a group desk option is open to you, so you know in advance whether you are splitting the party or handling it off the portal.