As the GetSharedDefaultFolder method description states:
This method is used in a delegation scenario, where one user has delegated access to another user for one or more of their default folders (for example, their shared Calendar folder).
Is there a way to get shared calendar appointment items without being a delegate of the person?
For example, I could do an File -> Open -> Open User's Folder to see people's busy/free status for about anyone or mail box. But apparently this permission level won't enable the GetSharedDefaultFolder() method.