On https://glass.google.com/myglass, apps that require a login do it from the website before installing the app. How can I create an app that requires a login like this? I can't find anything in the documentation about it. Also, how can I test the app since it would not be in myglass?
Although Google has worked with some partners to get GDK-based Glassware in MyGlass that use auth, there is no public method to do so yet. This is a frequently requested feature, and you can expect that once the GDK leaves Developer Preview, it will be available.
Until then, you will need to test your app by sideloading the app onto Glass. If you're testing for yourself, you can hardcode the auth into the app, and many people have hacks that use QR codes.
Keep in mind that this only holds true for GDK Glassware. Anything built with the Mirror API has authentication as part of its web-based initialization which you can trigger without having to go through MyGlass.