From http://answers.microsoft.com/en-us/mac/forum/macoffice2011-macoutlook/add-new-attendee-to-exhisting-meeting-causes-all/cb5b32ca-42cf-431e-93f2-6cd3c7b42a93?db=5&auth=1
These steps should allow you to send your new meeting invite/update to only your newly added delegates.