You may join areas in the Designer by:
Drawing first all the areas you wish to merge.
Selecting all the areas to merge.
Clicking the Join Areas button at the top right of the screen.
How do Joined Areas behave when selecting places?
They behave exactly like any other area. Capacity is defined in the Designer, and is shared across all shapes.
The only differences are found in their visual feedback:
When hovering, pressing or selecting an area, all of the others get this state as well.
Labels and the selected checkmark with the chosen quantity isn't displayed.