Add a Linux User

In order to add a user you should create their group first or decide which preexisting group you will assign them to. If you’re adding a new group for the user:

Add the group:
groupadd $group

Then add the user
adduser -g $group -d /home/$username $username

This will add a user with the username $username and it will create the folder: /home/$username and then automatically assign them to the group $group.

If you’re managing a client’s server it’s common to create a second root user. This will allow you to login with root privileges even if they change the primary root users password. To create a second root user:
adduser -g root -d /home/$username $username

Then edit the file /etc/passwd
You will see a line that looks like:
$username:x:777:778:$username:/$username:/bin/bash

The file is delimited by a :

You should update the 3rd and 4th entry so in the example above you would change 777 and 778 to 0
$username:x:0:0:$username:/$username:/bin/bash

Then save the file. This will ensure the new user has root privileges. It will also allow you to login and reset the root password if the client locks themselves out of the server.