To list users:
getent passwd
To add a new user:
sudo adduser newuser
To change a user's password:
sudo passwd username
To change a user's username:
sudo usermod -l newUsername oldUsername
Note that this doesn't change the home directory. To change the home directory to the new username run:
usermod -d /home/newHomeDir -m newUsername
To delete a user:
sudo deluser newuser
To add a user to a group:
groups newuser
To add a user to sudoers group:
usermod -aG sudo newuser