Signed-in users can also find this guide, with screenshots, under Help in the app.
The area stays with its own building. Bookings from the other building count toward that building's limits and show on its front desk; patrol at the garage's building sees every car.
Open Parking, open the area and select Edit.
Under Shared with other buildings, tick the buildings that may use it.
Select Save. The area now appears in those buildings' booking and visitor forms.