New iCal invitees without updating entire group

You can invite new contacts one at a time. This invitation does not go to existing invitees. Follow Apple's instructions in Send invitations in Calendar on iPhone

A change to the details in the event should trigger an update to existing invitees. If you delete an invitee they will be notified that their invitation has been withdrawn. This update will not go to anyone else.

cwittman wrote:

Thanks David, to clarify, are you saying that I must only add 1 at a time in order for the entire group not to get re-invited each time?

It's my experience that the invitations work independently. You can add them one at a time or as multiples and it should not affect the existing invitations. I have not used a group of invitations in quite the way you are managing.

If there is an unexpected behaviour, as a last resort you can alert your group about that old problem of user error.