You can respond to event invitations in two ways:
Through the email invite:
Select from the given options of Yes/Maybe/No and your response would be sent to the person who sent you the calendar invite.
Through the event on the Calendar:
Select the new event on the Calendar and select from the given options of Going/Not Going/ Maybe.
We hope this was helpful! If you have any queries, you can reach out to us at firstname.lastname@example.org