If a large group wants to travel together, can I book them all in one go on my website?

Usually not as one single booking. Most airlines set a small limit on how many passengers can sit on one ticket order, and a party bigger than that is treated as a group, which airlines handle through their own group desks rather than through the live search that feeds your portal. So the booking screen your customer sees is built for a normal family or a small party, not for a whole coach load.

What your portal can do is split the party into several smaller bookings, one after the other. That works, but each booking is priced on its own at whatever is available at that moment, so the later ones can come out dearer than the first, and seats together on the plane are not guaranteed. For a handful of travellers that is rarely a problem. For a pilgrimage group or a company outing it is almost always better to go for a proper group fare.

The common way round it is a group enquiry form on your site, where the customer gives you the dates, the route and how many people are travelling, and it reaches you as a lead instead of a booking. You then agree a fare with the airline or your consolidator and put that fare back on the portal for the group to pay against. Whether that form and the manual fare loading are available to you depends on your plan and on the suppliers you are connected to.

Ask your supplier what the passenger limit is on each airline you have connected, and decide whether a group enquiry form belongs on your site so you stop losing those bigger trips.