Can I book a large group travelling together on my portal, or do I have to arrange that myself?

No, not as one single booking in most cases, because airlines and hotels treat a large group as a different kind of contract from an ordinary reservation.

The flight APIs that feed your portal have a limit on how many passengers can go on one booking, and that limit is set by the supplier, not by us. Once your party is bigger than the limit, the fare you see in a normal search is no longer the fare the airline will hold for you. Airlines run a separate group desk that quotes by hand, usually with a deposit, a deadline for passenger names and its own rules about changes. The same applies to hotels once you want more rooms than the supplier allows in one go.

What your portal can still do is useful. Your staff can split the party across several bookings in the same session, which works well when everyone is happy to travel on the same flight at the going fare. They can send the customer a quote, keep every booking under one customer record, and add notes so anyone in your office can see how the group fits together. If you obtain a group fare offline, you can record it in the portal as your own booking so the sale and the payment sit alongside everything else.

Check the passenger limit your flight supplier allows on a single booking, and ask the airline group desk what they need from you before you promise the customer anything.