It's not just price, but flexibility. Unlike individual tickets, you generally have the flexibility to substitute a passenger up to X days before initial travel and you also generally do not have to pay the entire sum at the time of booking (generally a significant perk when you are booking for a group where you need to go out and collect).

Group bookings are not the same as individual bookings and it is critical to read the t&c of the booking very carefully.

In this case, I would not have done a group booking. If you don't need the flexibility on passengers and payment, you are paying a bit more for significant flexibility you don't need.