When you get an invitation to an event, it appears on your calendar.
To reply to an invitation in Calendar:
In the calendar grid, click the event.
In the dialog box, select your response. Or, if you’d like to include a short note with your response, click More details, select your response, type your note, and click Save.
You can change your response at any time by clicking More details > Add a note or change your response.
You’ll also get an invitation in your email. If you're using Gmail, you can respond to the invitation by clicking RSVP in your list of messages, or by clicking a response option in the message.