Commands that are used to administer a computer must be executed by an administrator user or the root user. If you get a message such as “permission denied,” the command probably requires root privileges. You can execute one command at a time as the root user, by beginning each command line with the sudo command.
You must be logged in as an administrator to use the sudo command. If you haven’t used sudo recently, you’re prompted to enter the administrator password.
For example:
sudo passwd newuser