One of my end-users (UserA) is getting the following error message when he
tries to share a calendar with another user (UserB) in the same domain:

"Calendar sharing is not available with the following entries because of
permission settings on your network"

Now I've tried creating a test calendar and can successfully share it among
both of the users - no issue for me.  We're all using Office 2007.  
Permissions look good on the calendar and when compared to other calendars.  
It's just odd that UserA cannot share to UserB.  User A can share calendars
to me, and vice-versa.

Any suggestions would be appreciated.

Thanks in advance,

Brian