Mostly, Nick is a narrator who, with his selection of details, is showing us something, rather than telling us something, and we can make some inferences from what we are shown in the text. But he does tell us something about all of these people in Chapter III. The motivations of the people who attend these parties are essentially two: they want to party, and they are attracted by the rumors that swirl around Gatsby.
In Chapter III, Nick says he is one of the few people who is actually an invited guest, and that most of these people went to Gatsby's parties uninvited. They often "came and went without having met Gatsby at all" (45), and it seemed that "they conducted themselves according to the rules of behavior associated with amusement parks" (45). Then Nick says, "They came for the party with a simplicity of heart that was its own ticket of admission" (45). In one conversation between Nick, Jordan, and Lucille, one of these "guests," Lucille says, "I never care what I do, so I always have a good time" (47). All of these people have come for a party, to be wined and dined, to be entertained and amused.
In addition, Gatsby himself is a draw for these people, who are curious about him. He is a mysterious figure, the subject of many tantalizing rumors. Nick shares some of them, showing us how all of these party-goers are also attracted by a mystery. In Chapter III, one guest informs Nick she heard Gatsby had once killed a man, and Lucille says he was a German spy in the war (World War I.) One of the young ladies, in Chapter IV, reports that Gatsby was a bootlegger and a nephew of von Hindenburg. All of these are fascinating stories, far more interesting than the lives these guests are likely to lead, and most of us can see the appeal in attending a party hosted by such an interesting figure.
What we understand from Nick with his detailed list of attendees is that these are people who know nothing at all about Gatsby, but are lured by the promise of a party and the chance of a glimpse of this mysterious and wealthy man. This seems rather sad, but the fact is that Gatsby cares nothing at all about any of these people. The parties are meant as way of attracting Daisy, almost a stage set for her to appear on, as well as a sort of conspicuous consumption that Gatsby hopes will impress Daisy, to gain her love.