In chapter 3, what is revealed when Nick says that people aren't actually invited to Gatsby's parties, it is more that they just show up?

Caterer trucks would be seen arriving at Gatsby's house and people would "crash" the party. Gatsby did not invite people, but word of mouth drew people to it. The "guests" had no problem relieving Gatsby of his food and liquor, all the while gossiping about their host. Gatsby was cognizant of human curiosity and took advantage of it to advance his own purpose, which was that eventually Daisy would probably show up at one of his parties.

Gatsby kept himself apart from the guest. The party itself was merely a tool he used in getting what he wanted. He had no desire to socialize with the attendees. Nick was one of the few people he ever literally invited.