I believe the DVC party is held on all cruises. It's not the kind of thing you will hear about or get "invited to" in advance, when you receive your first Personal Navigator in the cruise terminal it will just have it listed in there with a time. It's while the ship is still docked before the emergency drill. Have your membership card handy just in case, the crew at the venue will have a list of all/most DVC members on board, on my cruise my partner and I weren't on there for some reason but showing our membership cards cleared it all up. I find the party a little silly, it's more a sales pitch for people to add points with some little games (eg. guess the movie from the song) and freebies and free cocktails to entice people to come along. I went to the DVC-only party and also accompanied some friends to the sales pitch on my last cruise, and I honestly preferred the merchandise they gave out to the non-members! The party gave out some USB drives, the normal pitch was giving out DVC-branded magnets, pins and a nice beach/snorkel satchel.
I'm not exactly sure how
Castaway Club works if the cruiser has only taken member cruises in the past, but the way it works for standard cruises is on your second cruise and beyond you will have a gift (usually a bag of some sort) waiting in your stateroom. The style of the bag corresponds with whatever CC tier you are (eg gold members generally get a gold bag). I think gold and platinum members also get a photo frame with their bag. Someone else may be able to answer if member cruises count toward your CC club status, if they do then there will be a bag waiting for you on embarkation day! (It's a limit of one bag per stateroom, even if there's multiple people in a room that have cruised before it's just the one bag).
