If you have attained Platinum (5 cruises or 50 days) or Elite (15 or 150 days) status with Princess' loyalty program, or are in a full suite you will receive a card from the Captain circle host that outlines the days the lounge is open (usually every day except the nights the Captain Circle Party is).
Here is what we had for a drink special and complimentary pre-dinner canapes while we sailed on a 10 day Emerald Cruise. Drinks are now $5 fleet wide.
Beef Steak Tartar with Melba toast - Chivas Regal Rob Roy
Smoked Salmon with Toast Points, Onion rings, capers and lemon - Chairman of the Board
Authentic Mexican Salsa with tortilla chips - Mojito
Wheel of Stilton with port wine, walnut baguette - Sandleman's Port
Herb-Marinated Goat's Cheese with Crisp Rosemary Flat bread and Tomato Bruschetta - Brezza Marina
Shrimp Fountain with cocktail sauce - Cosmopolitan
Assorted Sushi with soy sauce, wasabi and pickled ginger - Japanese Slipper
Note there are usually cheese, veggies, crackers and olives also served.
I have seen it where there is someone at the door checking names to ensure you are allowed in, and I have seen it where there was no one at the door checking. There normally is a sign stating it is the Elite/Platinum Cocktail Party and/or Private Party.
I often get asked, can I bring my friends who are not Platinum? Well technically the answer is no. But do I see it happening, yes. I don't think they would have a problem with bringing one or maybe two people in but like all the rules on the ship, if everyone didn't follow them there would be no sense in having them. So if you hate if people don't follow dress code in dining room, save seats in theatre, throw towels on lounge chairs to hold for hours, and send their kids into the adult pool then you know what the answer is.
We have found that on our last couple cruises we rarely even go to the cocktail hour as we find the times don't fit well with our dining time of around 7:45 or 8 pm. I don't want to dress early just to go there and then when it closes down at 7 I have to wait around even longer for dinner. Also now that they have raised the prices up to $5 it really is not enough of a savings to entice me.
One thing we have noticed on some ships (note may not be available on all ships) is that you can order any of the drinks that are on the Elite Menu, so even though it is Rob Roy night you can order a Mojito for the same price even though it is not Mojito night.