Attempting to send another Exchange 2007 user a request to share calendars results in the error:
“Calendar sharing is not available with the following entries because of permission settings on your network.”
This one was so simple it’s obnoxious. Instead of typing in the user’s name in the To: field, click the To: button and select their Exchange address from the Global Address List. I suspect that I simply allowed auto-complete to fill in the user’s name when I typed the first few letters and it was not actually the Exchange account but another of the user’s email accounts that I had previously named with the same username.