The guests at Gatsby's parties include all the "beautiful people" of his day. Gatsby's aim in entertaining, as with most things, was to present himself as being highly desirable and influential in social circles. He entertained lavishly and invited important people who would be able to help him achieve his goals of making himself well-known.
From West Egg came the Poles and the Mulreadys and Cecil Roebuck and Cecil Schoen and Gulick the State senator and Newton Orchid, who controlled Films Par Excellence, and Eckhaust and Clyde Cohen and Don S. Schwartze (the son) and Arthur McCarty, all connected with the movies in one way or another.
In addition to the persons Gatsby invited to come to his parties, there were always plenty of others who came without an official invitation. These persons, who came for the excitement of associating with the official guests, were generally not as prosperous or well-established in society, but they were eager to contribute to the festivities and to partake of the liquor and to share in the general atmosphere of uninhibited merriment.
I believe that on the first night I went to Gatsby's house I was one of the few guests who had actually been invited. People were not invited - they went there...Once there they were introduced by somebody who knew Gatsby, and after that they conducted themselves according to the rules of behavior associated with an amusement park.
Gatsby's parties are attended by the famous and the nouveau riche.
In Chapter 4 of The Great Gatsby, Fitzgerald provides vignettes of the time period, a period in which the acquisition of money by any means was often justified. Those who attend Gatsby's parties are not the wealthy old families of East Egg but are instead those of questionable reputation, such as Ripley Snells, who is there "three days before he went to the penitentiary" and James B. ("Rot-gut") Ferret. He and the de Jongs and Ernest Lily have come to gamble. Nick narrates that when Ferret "wandered into the garret," it meant he "was cleaned out" and on the stock market the next day the Associated Traction "would have to fluctuate profitably."
Further, Fitzgerald satirically provides a list of names, many of which are ridiculously piscine--"the Fishguards, the Hammerheads and Beluga..."-- or absurd in sound, such as Mr. Albrucksburger and Miss Haag, his fiancee. There is also Miss Claudia Hip, who is accompanied by her chauffeur. One guest has the tendency to stay on so long after parties that he has become known as "the boarder": Mr. Klipspringer.