You have multiple mailboxes and need to make a bulk change to all calendars, for example you would like for one user to be able to access all user calendars.
Microsoft have released a new tool called exfolders.exe. The download also includes a read me file with clear instructions on how to get started.
If you’re using Exchange 2007, download PFDAVADMIN. PFDAVADMIN was replacd by exfolders.exe for Exchange 2010.
DOwnload exfolders here