"Calendar sharing is not available with the following entries because of permission settings on your network' Exchange 2010

Environment: 

Exchange 2010, Outlook 2010

Issue: 

Users are unable to share the Calendar and receiving the message from Outlook and OWA

Resolution : This could be due to the Cached User name, Try renaming the .PK2 file and restart outlook or Use GAL to select the contact and then share the Calendar.