To list all users you can use:
cut -d: -f1 /etc/passwd
To add a new user you can use:
sudo adduser new_username
or:
sudo useradd new_username
To remove/delete a user, first you can use:
sudo userdel username
Then you may want to delete the home directory for the deleted user account :
sudo rm -r /home/username
(Please use with caution the above command!)
To modify the username of a user:
usermod -l new_username old_username
To change the password for a user:
sudo passwd username
To change the shell for a user:
sudo chsh username
To change the details for a user (for example real name):
sudo chfn username