Office 365 (Outlook 2013 / 2016 for Win) - How Do I Add Additional People to an Existing Meeting?
If you have scheduled a meeting in Outlook that everyone has already accepted and you would now like to add additional attendees, please follow the steps below. This will ensure that the meeting request will only go to the new attendees and will appear on their calendars once they accept it. Do not simply forward the meeting to the new attendees as it will not appear on their calendars.
1. Open up the Meeting from your calendar.
2. In the To: field add in the new attendee (you could add them to the CC field as well).
3. Click Send Update.
4. A dialog box appears where you have the choice to send to only the added attendee(s) or to everybody as shown in the following screenshot.
5. Select OK.