We started moving mailboxes to Exchange 2013 and deploying Office 2013 around the same time and Office 2013 users are having issues editing their own entries in the shared calendars:
We have users experiencing issues with shared Calendars in Outlook. Unless they are the owner of the calendar they are not able to edit, add or delete any information on the calendar. This includes information that they placed there prior to the exchange upgrade. Most are set as “Author” on the calendars and should be able to edit their own information but they are not allowed. I have had the owners remove and readded the users and reset their permissions with no success.