When you use Outlook to share a calendar with another user, you might get one of the following errors:
Calendar sharing is not available with the following entries because of permission settings on your network
You Cannot request to share calendars with the following people because of permission settings on your network
This typically happens when users type the recipients’ email address or use the address from their cached addresses. When this happens, clear the cached entry for the user, click the TO button and select the intended recipient(s) from the address book.