The thing is, even if we replace the time with “RSVP,” that slot can only be booked once. We already have a request for allowing multiple appointments in the same time slot. Feel free to upvote it here — Multiple appointments for the same time slot.
Additionally, there’s a request for an RSVP feature for our Event Calendar app, which would be a better fit for this situation. You can support that idea here 
— Add a RSVP feature.