I think people are confusing two different parties. The past guest party is by invitation only and is held in the auditorium where the nightly shows are held. It's about an hour long, maybe a little more. The waitstaff serves ready made drinks and hors d'oeuvres. There's usually a band and you can dance up on the stage. There's a little talk about Carnival and a short film or slide show of what Carnival's history and plans for the future. The captain will introduce the some of the crew members.
The Captains' party is held on formal night just prior to dinner and is open to everyone. The captain welcomes you on board. There's a band and dancing on a small dance floor. There are hot hor d'oeurves. and you have the option of the ready made drinks or using the bar in the room.