The primary Room is the Room that your guests will appear in when they first enter your Space. It is also where you reappear if you select Respawn from your personal menu in the toolbar.
To change the primary Room of your Space, open the Mapmaker. Select the Room name in the Room tab in the right panel. Select the three dots to the right of the name, and select Set as primary.
The primary Room will then display a house icon to the left of the name.
Note: Every Room requires at least one Spawn tile.