Outlook 2007 SP3 is supported with with April 2014 Public Update KB 2863811
For you as a System or Exchange administrator or Consultant you might consider some things before migrating to Exchange 2013.
Error while preparing to send sharing message.
Also, if you try to check the permissions on the calendar, you may receive the following error message:
An error occurred when setting schedule permissions.
Solotion Microsoft: Using OWA
It works for share the calendar and you can set editor calendar permissies. All other permissions are not availible
With Powershell you can set the permissions
Add-MailboxFolderPermission –identity (username+’:\calendar’) –user testuser –Accessrights PublishingEditor
Upgrade your Outlook Client to 2010 or 2013