Which Backend Framework is Best for a Simple Web App with User Auth and CRUD Functionality?

Ruby on Rails is a solid choice! It can easily manage CRUD operations and handle APIs. You'll need to set up some user group logic manually, perhaps by creating a Group model to associate users, but it's definitely doable.

Consider the MERN stack! It allows you to use JavaScript across the board, simplifying development. It’s popular among big companies, plus performance is impressive compared to others.

When picking a backend framework, you should consider two main options: choose the one you already know or the one you're eager to learn. Most backend frameworks can handle what you need, so go with what you're comfortable with.

Given your requirements, I’d suggest Django if you’re into Python. Its mature ecosystem works well with authentication and prevents headaches related to session management.

If you're a fan of C#, ASP.NET is a great option. It's robust and suits your needs perfectly.