When a user follows your subscription calendar, all the events contained in that subscription calendar are added to their personal calendar. Before an event takes place, the calendar service they are using (e.g. Google, Apple, Outlook, etc.) will often send them a push notification reminding them of the upcoming event.
While these reminders can be extremely useful, they are entirely controlled by the calendar services, and thus AddEvent cannot guarantee them. In the past, calendar services have even been known to block these push notifications for calendars that their users are following, so as to prevent these calendars from spamming their users with notifications (something AddEvent would never do).
Please note that when you enable RSVP functionality for your events, you can enable customized reminder emails for your attendees that are sent out before your event takes place. AddEvent does guarantee these reminder emails since we send them out to the email addresses your attendees have provided in your RSVP form.