You can send email notifications to event guests when you create or edit an event. In some cases, you might not be able to change whether or not guests get notifications. Even if you don't send email updates to guests, changes you make to events will still be reflected on their calendars.
Guests of past events
Google Calendar won’t send updates to guests of past events.
Guests who turn off notifications
Google Calendar users can change their email notification settings. Guests who turn off notifications won’t get invitations or updates you send.
Guests who don’t use Google Calendar
Google Calendar always sends notification emails to guests who don’t use Google Calendar.
Guests who don't want to automatically add events
Guests can choose not to automatically add events to their calendar. In this case, Google Calendar emails an invitation, even if the event organizer chooses not to send one.