You can reply to an event invitation or a shared calendar invitation in Calendar. You may also be able to reply via email or in a notification. If the invitation is for an event or a shared calendar in a calendar account that’s set up on your other devices, you can also reply to the event using any of those devices. If the invitation is for an event or a shared calendar in an iCloud account, you can also reply using iCloud Calendar on iCloud.com.
If you selected “Badge app icon” in Notifications preferences, the Calendar icon in the Dock has a red badge that shows the total number of invitations and other notifications you have for events and shared calendars.
Click the Notifications button 
in the toolbar.
The Notifications button shows how many invitations you have. If you don’t have any invitations, it isn’t available. A circled gray number next to a calendar name in the calendar list shows how many event notifications are pending for that calendar.
Do one of the following:
For an event invitation: Click Accept, Decline, or Maybe.
For a shared calendar invitation: Click Join Calendar or Decline.
Do one of the following:
Click Accept, Decline, or Maybe in the email.
Move the invitation to Calendar by opening the included event attachment. To reply to the invitation in Calendar, click the Notifications button in the toolbar, then click Accept, Decline, or Maybe.
In a notification with buttons on the right: Click Accept or Close for an event invitation. Click Join or Close for a calendar invitation.
You can also click the left side of the notification to reply in Calendar. See “Reply to an invitation received in Calendar” above.
In a notification without buttons: Click the notification, then reply in Calendar. See “Reply to an invitation received in Calendar” above.
In Notification Center: Click Notifications at the top of Notification Center, click the notification, then reply in Calendar. See “Reply to an invitation received in Calendar” above.
You can turn off all notifications for calendar events and shared calendars in Alerts preferences. You can change other notification options in Notifications preferences.
Note: For events in an iCloud calendar, you also receive an invitation in Mail, so you may see both a Calendar notification and a Mail notification.
If your declined events disappear from Calendar, and you want to see them again, choose View > Show Declined Events.