Group booking allows multiple customers to schedule same time-slot. It is most useful for scheduling Classes, Events & Work-groups. Group booking works at service level. In service, there is a "Capacity" field that allows number of bookings allowed for that service at a particular time.
Administrators can add a new customer to an existing booking by clicking on option's image against booking on "Day View" and selecting "New Booking". Administrators can add multiple customers to an existing Booking/Appointment/Event regardless of settings.
To understand scheduling a fixed day event, read here