Why can't other accounts login to the hosted app?

The Spotify Web API allows only up to 25 users within each app. On top of that, the users who will be using the app must be registered in the Spotify Developer's Dashboard manually.

This means that the details of the users using the app must be known beforehand. Since it's impossible to determine who will login to the app beforehand, we're restricted to using only known credentials.

This is why I have provided credentials for the MS Engage evaluators to try out the live app.