ADDING ADDITIONAL USERS TO WP SITE

I want to set up someone as an additional User with their own password so that they have access to make modifications to my site and the custom theme by rewriting some of the code. Would I do this through WordPress or must I do it through my hosting site? I’m very confused by my hosting site’s FAQ page description of FTP User, Database User, and MyPHPAdmin. I’m not sure what the differences are between these three things and what I need to provide to whoever I designate as an additional user.