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.
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:
Make sure you're in online mode first. In versions of Outlook before 2003 this was fairly obvious, but with cached mode it's a little more tricky because while in cached mode, Outlook might place you in offline mode without explicit action on your part. Look at the File menu and make sure the “Work Offline“ option is not checked
Go to Tools | Options | Other | Advanced Options | Add-In Manager and make sure the Exchange Extensions ones are checked