The latest release of Infoware's MasterCal Calendar Tool (released December 12, 2008) now allows Administrators to select and process a 'one-off' list of Attendees for each individual MasterCal appointment. Simply select the new MasterCal Tab on the Appointment form and display your Firm's Global Address List to select Attendees.
Setting Attendees on an item will over-ride the Calendar folder's default Attendees list - only the Attendees listed on the MasterCal form will receive updates for this Appointment. If an Attendee is removed from this list then MasterCal will delete the appoinment from this Attendee's Calendar while still pushing updates for the current appointment to the remaining/new Attendees in the list.
If you do not set Attendees on the Appointment item itself then MasterCal will use the default Attendees list in the Calendar folder's Description field.