Can I book a large group on one flight through my website or do I have to arrange that with the airline myself?

No, in most cases a large group cannot be booked in one go through a normal flight search on your website, and you will still need to arrange it with the airline or a group desk. Flight booking APIs follow the airline's own rules, and airlines limit how many passengers can sit in a single booking record. Anything above that limit is treated as a group and handled outside the live booking flow.

The reason is that group travel is priced differently. A group fare is usually quoted by hand by the airline's group department, held for a period against a deposit, and the passenger names are added later. A flight API works the other way round, showing live published and negotiated fares that must be ticketed almost immediately with full passenger details. The two processes do not fit together, so the API simply will not return a group quote.

What you can do is split the party into several smaller bookings that each stay within the airline's limit. That works, but be honest with your customer about the trade offs. Each booking is priced on its own, so the fares may not match, seats may not be together, and if one booking fails you are left holding part of the group. Some suppliers also offer a group enquiry option that passes the request to a human desk rather than returning an instant price, and whether you have that depends on your supplier and your plan.

Before you quote your next group, check which of your connected suppliers accept group enquiries and agree a simple manual process for anything larger than a normal family booking.