Google has added another new handy feature on GMail. This time, they integrated Google Calendar and GMail by adding invitation link when you compose a message. It will open up a dialog just like the screenshot below and then you can start inviting your friends for your future activities. The friend list will be taken from your TO/CC/BCC list.
I think it's very useful if you are using Google Calendar to manage your daily activities. By integrating this invitation menu into GMail, it saves user's time to open up Google Calendar. Very nice Google