A group booking most commonly refers to the buying of ten or more theatre tickets in one transaction and with sequential seat numbers. It is usual for there to be a discount for purchases made this way so that a group booking for ten people costs less than ten individual tickets would do separately.
Group bookings can refer to other multiple reservations made in one transaction, such as...
Variations on group bookings include a family ticket which is usually for a specified number of adults and children, and advance booking which offers a discount for buying tickets a minimum time in advance of the event or visit.
References
Group booking Wikipedia(Text) CC BY-SA