Back in the day you could set the user ID below 500 and it would automatically be hidden, now it involves a little more work. Open up terminal and run the following command:
sudo defaults write /Library/Preferences/com.apple.loginwindow HiddenUsersList -array-add username
You can add multiple users by separating them with spaces. If you have only one other account on the system, it will unfortunately show an "Other" button at the login screen. If you want to make the existence of the account even more hidden, you can remove the button by running this in the terminal.
sudo defaults write /Library/Preferences/com.apple.loginwindow SHOWOTHERUSERS_MANAGED -bool FALSE
Restart the system to make sure everything gets applied. If you have any problems with it leave a comment and I'll try to help you resolve it.