Shared Exchange mailbox not updating in Entourage on Mac

I have three users who have access to a shared mailbox on an Exchange server who also have their own email accounts.  When they receive a meeting invite or cancelation to the shared calendar account  it updates their own calendar but not the shared calendar.

I have emptied cache and forced a re-sync but that just makes all three Macs have the same data.  I hope I'm being clear.  I'd update them to Office 2011/ Outlook but the Exchange server doesn't meet the minimum requirements.

Thanks in Advance, Rick