Just press Ctrl+Alt+T on your keyboard to open Terminal. When it opens, run the command(s) below:
sudo nano /usr/share/lightdm/lightdm.conf.d/50-ubuntu.conf
Add the following line, save and close.
allow-guest=false
Reboot, and you're set.
Adding users to sudoers
If there is a sudo group, you can do the following:
$sudo adduser <username> sudo
This works because /etc/sudoers is pre-configured to grant permissions to all members of this group (You should not have to make any changes to this):
# Allow members of group sudo to execute any command
%sudo ALL=(ALL:ALL) ALL
Otherwise, you can add the user directly
$nano /etc/sudoers
add the following to the end of the file, and save:
username ALL=(ALL) ALL