I would recommend one of your first stops when you board is the customer service desk. Some of the tastings can book up very quickly. However if you find a tasting that you really want to do but is already full you might still be able to get in. My wife and I really wanted to do a wine tasting on a sea day but it was already booked by the time we attempted to do so. We went to the bar where it was being held about 15 minutes prior to the start time and asked the staff if we could also join. They promptly setup a tasting service for us without batting an eye. On our last cruise we saw another couple do the same thing and the servers didn't have a problem with them joining us either.