You can change the maximum occupancy of a booking to allow more people to be added to that booking without affecting the service.
Start in your calendar:
Click on the booking
Click the customers tab
Click the person icon
Click the + and - icons to change the maximum occupancy and click save.
Now that there is more available space within the booking, customers can either book through your online booking tools or you can add a customer to the booking internally by clicking on the Add customer button and entering their details.
Note: changing the occupancy of a service under the service menu won't affect the occupancy of any existing bookings for that service. A booking will honour the settings of the service at the time the booking was made.