I have had success both ways. When my in-laws had booked a cruise separately from me, and I joined at the last minute, I was able to be added to their tasting right when we got on board.
During another cruise, we weren’t able to get availability when we got on the ship, but they said to go and talk to the bartender right when it opened, and they were able to accommodate an extra person (in 2019). And yes, we definitely paid!