Update: Organizations can remove the option for 'do not send response', forcing recipients to send responses. See Remove Do Not Send a Response option in Meeting Requests for more information.
I received a question from a user who wanted to force people to respond to meeting requests:
Is there a way to force people to send an accept message, when the appointment goes into their calendar? If I add an event (accept) or meeting from an invite won't the sender be notified that particular person has accepted?
The organizer can't force anyone to accept a meeting or send a response. If this is a problem, company-wide training on the appropriate way to use meeting requests and how to use the responses is called for.
When an invitee accepts or declines, they have the option of sending a response. If they choose not to send a response, there is nothing you can do using Outlook's native features. If a user chooses not to send a response, it won't be sent and tracking won't be updated in outlook. Admins or managers would need to compel the invitees to send responses and could do so using custom forms or VBA. If the organizer has permissions to free/busy, he can look in the scheduling assistant to see if the meeting was accepted (marked busy).