Everyone that you would like to share an app through Preflight must be a "User" on the project for that app. So, you can add a user who may only have "viewing" permissions, but as long as their Adobe ID is part of that project, that app will appear in their Preflight app when signed in with their Adobe ID.
Basically, Preflight is associated with the user's Adobe ID. Whatever Project is associated with a given Adobe ID will appear when that Adobe ID is logged into Preflight.
Hope that helps!
-Tommy