If you were invited to an appointment, whether in the event detail page (Pic.1) or in the notification detail page (Pic.2) you can propose yourself, or communicate that you cannot, (Pic.3) and wait that the author can choose you as a candidate. You will receive three types of notification:
1) He/She accepted this candidacy : his brother has chosen you! It just has to go on preaching !!
2) Accepted another candidacy : You have come too late. Maybe next time.
3) Event canceled
In the appointment details page, you can also revoke his candidacy (Pic.4,5). If your application had been confirmed, then it will automatically notify the author that you are unable to attend