In any Exchange environment, Outlook will frequently return this error when you try to share a calendar or some other folder:

The problem almost never has anything to do with permission settings on the network, and only tangentially has to do with permission settings at all. 90% of the time, you can fix this error by selecting the recipient of the sharing invitation from the Global Address List, rather than from Outlook’s Autocomplete Cache or from your Contacts. If that fixes it, delete the recipient from your Autocomplete and from your Contacts, and you shouldn’t have that problem again.

Until you do.