There is a known issue in Outlook 2013 where the user receives the error message "You cannot respond to a meeting without an organizer" when selecting "Send the Response Now" to accept a calendar invitation. The resolution is to switch from online to cached mode
1.Once in Outlook 2013, click the File button in the upper left hand corner.
2.Click Account Settings, a drop down menu should appear, click Account Settings again. A window like the one in the image below should appear.
3.Highlight the email you would like to change settings for by left clicking on it, click Change afterwards
4.Check the "Use Cached Exchange mode" box. You can change how long offline mail is available to you via the slider below the "Use Cached Exchange Mode" box.
5.Exit and restart Outlook 2013.
6.If you wish to disable Cached mode, repeat all the steps and uncheck the box. Exit and restart Outlook.