Soooooo, yeah. To-Do List Reimagined isn't a real app, per se. But it can help you understand how to integrate your app with Azure Active Directory in a realistic way. In the sign-up form, try typing in an email address of a user with an Azure AD account. Then sign in as that user, and get started adding some tasks to that user's to-do list.
Once you've explored sign-up and sign-in, try sharing a task with other users in your company, and watch TDLR take the company by storm.
Finally, click the Admin link to login as an admin of your company and view a list of users that have been assigned to the app. As you add/remove users, TDLR's user store is automatically updated.
We're here to help with all your questions about integrating with Azure AD. We've already posted a lot of good stuff though, so check out our documenation first!