The Console - Learning Raspbian - page 104

You can add users to your Raspbian installation using the adduser command.
To do this, run adduser followed by the username of the user that you want to
create. Because adduser is a system command, you will need to add sudo before it.
Creating a user
You will be asked a couple of questions by adduser , including the username and
password. If you don't want to provide your name or phone number, simply press
the Enter key. The adduser command will also create a new group with the same
name as your new user.
You can easily see which users you have in your Raspbian system by running the
following command:
cat /etc/passwd
The /etc/passwd file contains all the users that are in Raspbian.
addgroup
Sometimes, you need to create a new group in order to organize your file
permissions as required. To do this, use the addgroup command.
Creating a new group