Add a message to the login window

You can add a string of text above the list of users in the login window. To do this, start by opening up Terminal. This is located in the Utilities folder, inside the Applications folder. Once it has loaded, paste in the following line and press return:

sudo defaults write /Library/Preferences/com.apple.loginwindow LoginwindowText "Hello There"

Replace "Hello There" with your text. To remove the text, type the following line:

sudo defaults write /Library/Preferences/com.apple.loginwindow LoginwindowText ""