Reply to invitations in Calendar on Mac
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 how many pending notifications you have for invitations and changes to events and shared calendars.
Reply to an invitation received in Calendar
In the Calendar app
on your Mac, click the Notifications button 
in the toolbar, then click New.A number on the Notifications button shows how many new notifications you have for invitations and changes to events and shared calendars. The button doesn’t appear if you have no notifications.
A circled grey number next to a calendar name in the calendar list shows how many new notifications you have for that calendar.
Do one of the following:
For an event invitation: Click Accept, Decline or Maybe.
For an unwanted invitation from an unknown sender: Click Report Junk.
For a shared calendar invitation: Click Join Calendar or Decline.
Reply to an invitation received in a Calendar notification
In the Calendar app on your Mac, do any of the following:
In a notification with buttons: For an event invitation, click Close or Accept, or click Close or View. For a calendar invitation, click Close or Join.
You can also click the left side of the notification to reply in Calendar.
In a notification without buttons: Click the notification, then reply in Calendar.
In Notification Center: Click Notifications at the top of Notification Center, click the notification, then reply in Calendar.
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.
Reply to an event invitation received in Mail
In the Calendar app on your Mac, do one of the following:
Click Accept, Decline or Maybe in the email.
Open the event attachment to add the event to Calendar. Then, in Calendar, click the Notifications button
in the toolbar, click New, then click Accept, Decline or Maybe.
Propose a new time
After receiving an event invitation, you can propose a new time to the event organiser. You can propose a new time before or after replying to the invitation.
In the Calendar app on your Mac, do any of the following:
In Day or Week view, drag the event to a new time, then click Propose or Cancel.
In Week or Month view, drag the event to a new day, then click Propose or Cancel.
If the event is on a calendar service that tracks availability and lets you add a comment to the organiser, you can double-click or force click the event, click Propose a New Time in the event info window, then select one of the listed new times.
To cancel a proposed time, drag it back to the original time.
How the organiser is notified about your proposed time change depends on the type of calendar service.
iCloud and most other services: You’re offered the option to send the organiser an email.
Recent Exchange services and some CalDAV services: The organiser can click the Notifications button
in the toolbar, click New, then find your proposal in the notifications list. A Calendar notification may also appear in the upper-right corner of the organiser’s screen, depending on the organiser’s Calendar settings in Notifications preferences.Some Exchange and CalDAV services: An email is sent automatically to the organiser.
Change your reply to an invitation
In the Calendar app
on your Mac, click the Notifications button in the toolbar, then click Replied.Find the invitation in the notifications list, then click a button in the invitation to change your reply.
You can also change your reply in the event info window. To do so, double-click or force click the event, click the My Status pop-up menu in the event info window, then choose a status. You can also add a comment to your reply that is seen only by the organiser.
If your declined events disappear from Calendar, and you want to see them again, choose View > Show Declined Events.