We use Exchange 2013 and Outlook 2010 at my work.  I had a few people trying to share their calendar to others in the organization.
When they tried this they received the following error message:

“Calendar sharing is not available with the following entries because  of permission settings on your network.”

Well I’m the network and Exchange admin and I know I haven’t intentionally setup anything to block sharing of calendars.  After a bit of playing around I found a strange but easy fix.

When you select the person you want to share the calendar with make sure you click the to button and select the person you want to share with from the global address list.If you simply start typing the users name in the to box you’ll get Outlook’s cached version of a user and that will give you the permissions error.

I found this issue to be present in Outlook 2010 as well as Outlook 2013.