If someone sends you an iCal attachment in an email, when you view the message you will be presented with a toolbar that looks something like this, depending on your theme:

When you click on the Accept Invitation or Decline Invitation button you will be prompted to decide how you want to respond to the event organizer.

If you choose to Send the response now a message saying you have accepted the appointment is sent to the meeting planner. If you choose to Edit the response before sending, a compose window will open so that you can write a personalized message explaining why you are or are not attending. If you choose Don't send a response or click Cancel in the dialog, no message will be sent to the meeting planner.

See Also

Calendar

Appointments

Scheduler