In the text box at the bottom type any notes you want the attendees to see in their email notification.
Click the Save button on the toolbar.
Note: Unless you uncheck Request Responses under the Attendees field, invitees will be sent a meeting request email message and the meeting is displayed on the calendar of the organizer and all invitees. Any time the meeting details are changed, attendees are automatically notified of the change, via email. You can uncheck Send Notification Mail on the Appointment Details page under the Attendees field so attendees do not receive an email message notifying them of changes to the meeting details.