Google Extension: INVITE ALL
This only works when using Google Chrome as your browser. If you don't have Google Chrome install it here, you'll be glad you did. Chrome has apps or extensions that can be added to the browser. Invite All (for Facebook) is an free extension that can easily be added. Click HERE to go to the install screen, click install and your done. You might need to close and reopen Chrome for the extension to work. Now when you go to invite facebook friends there will be a "select all" button at the bottom of the invite friends popup.