RSVP to an online invitation is done within the event invitation page that was shared with the guests, either by email or a direct link.
Click on the green RSVP button, then choose RSVP status and confirm. In some cases guests also need to fill their names and email addresses and indicate number of additional guests.
After RSVP is submitted, it is possible to change the RSVP status by clicking the green RSVP button again.