When you say new user, do you mean a new user to the FUEL CMS admin? If so, you can click on the "Users" menu item on the left in the CMS to create new users and assign permissions.
If you are wanting a separate new user unique to your application and outside of FUEL, that will need to be something you'll need to develop separately since it will be specific to your application.
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If you are wanting a separate new user unique to your application and outside of FUEL, that will need to be something you'll need to develop separately since it will be specific to your application.