Hi Glenn,
Have you tried readitems permission to the security group? To do this, via using PowerShell command syntax:
Add-MailboxFolderPermission –Identity shared:\Calendar -AccessRight ReadItems -User <security group>
For more information about Add-MailboxFolderPermission: http://technet.microsoft.com/en-us/library/dd298062.aspx
Moreover, if you need further assistance on this particular issue. I would like to suggest you post a new thread with detailed information on the forum. Our engineers will focus on the issue you encounter so that it can be resolved more efficiently.
Thanks for your understanding.
Best regards,
Shyamal