Auth0 seems like the most viable way to have full-fledged user management in AWS without too much hassle. Aside of being paid, it offers many advantages, such as social network logins or handy administration interface.

Authentication as a service. Instead of rolling our own user authentication flow for yet another time, it's better to leave the security concern to a service specialized in this field.

Devise handles user registration with low effort. Also is configurable in so many levels, from custom views to create-your-own controllers.

4 years of experience using Devise for User management, authentication, and authorization.

great and straight-forward authentication and user system via ruby gem.

External user authentication and authorization (roles & groups)

Open source user authentication in-a-can, what's not to like?

I use this occasionally if I need a quick auth setup.

For user authentication and signal-sing-on service.