I have created a WebApp in AGOL and have shared it with a Group that includes our client's ArcGIS Online Account. However, when I sent them the link to the map, they entered their login credentials and refused access because they are not a member of our organization.
Some articles said that to share a private app with members of different orgs you need to deploy it (and it will likely need custom widgets in the future), so I did the following:
1) Loaded the app into my Web App Builder 2.8. It launches successfully from within WAB.
2) Deployed the app to http://www.summitgis.epd-pgh.com/
3) Added the deployed app to our ArcGIS Online Account
At this point if you go to the URL, you get this prompt:

I logged in successfully, which is surprising - I thought I would need to register the app first!
4) Registered the app (redirect URIs: http://www.summitgis.epd-pgh.com/, https://www.summitgis.epd-pgh.com/)
5) Added the AppId to the config.json and replaced it in the server-side version.
Now - when I go to the link it won't load! I just get the blue screen and loading bars.
I'm really confused - I don't understand the purpose of registering the app, don't understand why adding the AppId is causing an error, and am unsure about whether I need to set up a proxy or not.