In order to mark a reservation as a group booking, in the reservation form change the booking type from Individual (default) to Group booking.
Enter the Group Owner Name which is obligatory to mark the reservation as for the group. Then, remember to save the reservation.
You can see all your group bookings using the filter in Reservation View. Select the dates and Filters, uncheck individual bookings box to see only group bookings.
Check also:
How to create a simple booking
What the reservation status mean and which ones are changed automatically?