Add a main contact to a booking
To assign a main contact to a booking, open the booking window.
Under the tab Booking click on the plus symbol 
beside the point "Main Contact".
The guest management will be opened, where you can select the guest you want to add from the guest list and set him or her as the main contact by a double click or by selecting the button "Add to booking" (bottom right).
If the main contact of a booking changes, you can edit it later on.
All changes in the guest management will be saved automatically.