If the people that are invited have the app they will receive a push notification if the event is cancelled (unless they have turned off push notification in the app or phone settings, or declined the invitation).
Those that do not have the app, but that have replied that they will come to the event, will receive a new SMS. Those that have not yet responded to the event, or declined it, will not be notified. Recipients that have previously registered their e-mail or at www.spond.com, will receive an e-mail.
If the person is only registered with an e-mail account, he or she will be notified if they accepted the invitation to the event, or it they have not yet replied.