shared calendar

We are having problems accessing shared calendars. Running SBS2000 with
Outlook 2002. Have set the permissions via user1's Outlook to allow user2
to access it (editor). User2 can bring up the calendar, but when you try
and switch to another day or month we receive the following error:

You do not have sufficient permission to perform this operation on this
object. See the folder contact or your system Administrator

If we go into Active Directory Users and Computers and give user2 mailbox
rights to user1's mailbox all is well, except that user2 has access to
everything, not just the calendar. Any help is greatly appreciated.

Thanks

David Kennedy