Users can be managed by an Administrator with appropriate permissions in Mautic by accessing the settings cog wheel at the top right of the screen, and selecting Users.
Create the user by completing all the mandatory fields, adding a signature if required, and assigning them a username and password. Always use a secure password for users with access to your Mautic instance.
It is also possible to set the default timezone and language for the new user, and to set the user to published (able to log in) or unpublished (unable to log in).
You will then need to provide the login credentials to the user to allow them to log in - ensure you do so in a secure way!