If you're using Outlook against an Exchange server, you can share folders between users. This can be done via a few different UI entrypoints - you could go to Tools | Options | Delegates and add a user, or you could right click on a folder and choose Permissions. The target user ('delegate') can then open that folder via File | Open Other User's Folder, or also via OWA by entering in the direct URL to the calendar - http://server/exchange/myboss/calendar.
What to do, however, if the permissions tab on the calendar folder is missing? I ran into this earlier this week, and some Outlook MVPs clued me in.
A couple of things to check:
In my case, my problem was the first one above. These folks are great.