Can I book a large group of passengers on one flight through my portal, or do I have to contact the airline myself?

Usually no, not as a single group booking. Almost every flight supplier sets a limit on how many passengers can go into one search and one booking, and group travel sits outside that limit. Group fares are normally negotiated by the airline group desk, with their own deposit rules, name change deadlines and payment terms, and those contracts are not exposed through the booking APIs that power a portal. So for a genuinely large party, you will still be speaking to the airline or your consolidator directly.

What the portal can do is handle the party as separate bookings on the same flights. You search as you normally would, book within the passenger limit the supplier allows, then repeat for the rest of the group. Each booking gets its own reference, so you keep an eye on all of them from your bookings list and the names and documents stay in one place. The catch is that seats are priced at whatever is available at the moment you book, so later bookings can land in a higher fare bucket, and the group will not be linked in the airline system.

The exact passenger limit, and whether any group handling is offered at all, depends on the supplier and on what your plan includes. Low cost carriers and full service airlines behave differently here, and some consolidators will accept a group request by email even though their API will not take it.

If group work is a regular part of your business, tell us which airlines and routes matter most and we can confirm what the limits are on the suppliers you are connected to before you quote a client.