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.
Bob and I are starting to be rather specific when we talk about the "DPS Preflight app" and "custom preflight apps" -- It's hard to hear the difference in capitalization, but the words the precede "preflight" are really important to us. Hopefully this helps others too. -- CF
The DPS Preflight app (it's in the App Stores) is one way to proof content (articles, banners, collections), cards, and layouts. You don't need to share this app, just have the other person download it. Once installed, sign in -- if your AdobeID is part of a project and has one of the permissions in the Content section enabled, then you'll see the project in DPS Preflight. If you're part of multiple projects, you can select which project's content you want to see.
If you're part of a project, but you don't have one of the Content permissions, you will not see the project.
If you are the master or have master admin access, you can see the project but cannot view the contents of the project. (There's a message to this effect.)
OTOH, if you build an app and enable preflight, you're making a custom preflight app (with your icon/splash and custom font support.) -- And you'd have to include the UDIDs for all testing devices in your mobileprovision file used in the app signing process.
Both DPS Preflight and a custom preflight app view the content in a project after you click the Preflight button in the Contents & Layout area. Wait for the progress bar to finish, then change views in the app to see the update message.