When organizing an event, it's proper to send out some form of notification (otherwise, no one will know that you're having an event and, thus, not show up). Some people still use that old-school mail thing that involves paper, stamps, and some other stuff - but c'mon, let's be honest here - this is the 21st century and we're geeks. It's a whole lot easier to grab your mobile and shoot out a few texts from the comfort of your couch, albeit highly informal.