Remember that every move Gatsby makes is calculated to draw Daisy to him. Invitations might make his parties appear exclusive and this might bar Daisy from returning to his life. This openness also was typical of the attitude of the 1920’s. It was a very open era and people could reinvent themselves every week if they wanted to. FSF paints a wonderful picture at these parties of the thin veneer-covering people that were scared, insecure, lost and friendless. It was a second Gilded Age with these parties starting in beautiful gowns and tuxedos and ending like frat parties.
As you read watch for Gatsby to create a world to lure Daisy. He is proving himself worthy of her attention. Read this blurb about Trimalchio. FSF begins the novel with an allusion to this figure and almost named the novel after him.
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.