Outlook 2011 and autoaccepting meetings
From Microsoft Support:


  If you have an exchange server, you can set that under the calendar
  settings in the ECP for each individual user. But it will only accept
  them tentative: "Automatically place new meeting requests on my
  calendar, marked Tentative"
  
  Its under Options > Settings > Automatic Processing (Right).
  
  Make sure your url reads http://exchangeserver.com/ecp not
  http://exchangeserver.com/owa


You can try something.  Use the OWA server, instead of the ECP server.  If that doesn't work, contact your system admin to make the change above.