To add a new user is simple enough. First of all you need to become root, you can do this by logging in with your normal user account then running the su command.
Once you are root you can add a new user with the adduser command, this needs a single parameter – the name of the account to add.
If you want to give the user ‘John Smith’ a login account. In the terminal you could run ‘useradd jsmith‘ to create a login for him.
Once this is done there are two more things you need to do – set the password for this account (they can change it themselves later) – you can do this by running ‘passwd jsmith‘
When you run passwd it will ask you for the password to set twice, to make sure you’ve typed what you think you’ve typed.
You can do it the graphical way in Gnome3 by going to Applications – System Tools – Preferences – System Settings – User Accounts.
There you have it, adding a user in Debian.