Outlook are releasing a new feature which will enable you to RSVP directly in the message list in Outlook on the web.
There will be an RSVP button, allowing you to respond without opening the meeting invitation.
After you click on RSVP, you will be shown a mini calendar to check for conflicts, and you can then accept, decline or tentatively accept.

Image courtesy of Microsoft
Let us know what you think of instant RSVP!