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

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 -G <groupname> <username>