To add a user you must use the sudo command. Here are the commands:
To add a user. NOTE: do not use the useradd command.
$ sudo adduser <username>
To see the options for adding a user try the man command.
$ man adduser
Here is a useful example of the useradd command. Why use useradd? It gives a few more options for special cases. To add a user, give her a standard home directory in the /home folder and specify the shell she accesses by default do this:
$ sudo useradd username -m -s /bin/bash $ sudo passwd username
Groups
You might also wish to create a new group for your users.
# sudo addgroup <groupname>
To add a new user to a existing group you would do this:
# sudo adduser <username> audio
To add an existing user to an existing group you can use the same command:
# sudo adduser <username> <groupname>
or
# sudo usermod -aG <groupname> <username>
