Is it possible the know which user viewed/opened a specific Outlook Appointment with the help of the EWS (Exchange Web Service) library ?
Ideal would be something like a history with the username of the user as well as the date and time at which that occured.
Exchange does not store such history.
It's possible only if you create a custom plugin for Outlook (Desktop Client) and make sure that everyone's using it (will not work from a mobile client).