To create a new Shutterfly user, all you need to do is a POST to the User API. This POST requires just a few pieces of information such as user name, email address and password.

One important note, though, is that you are required to ensure the user has accepted the Shutterfly terms and conditions, which can be found here: http://www.shutterfly.com/terms

Once the user has been created, you can treat it just like a pre-existing user, including getting user authentication and then uploading images to\ the user's account.