Add someone to an existing meeting

To add to someone to an existing meeting, open up the meeting details by double clicking the meeting on the calendar in Outlook. You can add the person like you normally would, e.g. typing their email address or adding them via clicking the "To..." button and searching them in the global address book.

Check their schedule in the "Scheduling" tab at the top to make sure they do not have a conflicting appointment. 

Then, click "Send Update". This will send an invite to that person while still keeping the other meeting details the same.