Where do I specify who can see which flatplan?

Click on the "settings" button in the top right corner and select settings from the menu. Note: Only Administrators can change permissions.

In the right column you’ll see a list of everyone subscribed to the flatplan. Check the box in front of each name to give them access to the flatplan. Uncheck to remove access. (People without access won’t see the project.)
Click "Update flatplan access" when your done to save.