Both are pretty much the right approaches, although if you guarantee to clients (client developers) that guest identifiers will always be unique no matter where they visit, I will go with
POST /guests/{guestId}
I usually use this:
POST /venues/{venueId}/guests/{guestId}
. , id 1 RSVP "" Id 1, URL- ( , , , / - RSVP):
POST /venues/1/guests/1
request body: {"RSVP", true}
, , ,
PUT /guests/1
request body: {"firstName":"John","lastName":"Foo"}
, !