Replying to a message with a meeting request is a new feature in Outlook, although how to do so is not immediately intuitive. You can reply to a message with a meeting request using one of the following three methods:
Select (or open) a message and click the Meeting button next to the Reply and Forward buttons.
Drag message to Calendar
Right click and drag the message to the Calendar button in the bottom bar of Outlook. Select Copy Here as Meeting Request.
Select a message and press Ctrl+Alt+R.