Gatsby's parties would be a perfect example to show you the separation between the two social classes that live in East Egg and West Egg; however, you would have to look more at the party that Daisy and Tom attend rather than the first party that appears in the novel would not really provide this information.
In the chapter after the first party -- Chapter 4 -- Nick lists a bunch of people who attend Gatsby's parties and although you do not really know which Egg the people are from, you can make general assumptions based upon their description. The corruption and deceptive naturla of these people is revealed.
Then, in Chapter 6, Tom and Daisy go to their first and last party at Gatsby's house. Tom and Daisy would be your sample of the East Egg people who do not really fit into the party -- they (especially Tom, a true East Egg, old wealth character) do not seem to feel as though they fit in with the other people. They feel that they are much more refined and well-groomed and seem to look down on those of West Egg who are more into socializing with those they do not know, getting drunk on illegal alcohol, and acting ridiculus.
There is also a scene in chapter 6 that shows the separation of the social classes in the book. Tom Buchanan, a man named Sloane, and a woman are riding horses and stop at Gatsby's for something to drink in order to cool off a bit. Gatsby is thrilled and honored that they have stopped at his house. He is almost childlike in his desire for their approval and their acceptance of him as an equal. Sloane is especially rude though. He looks down on Gatsby the whole time, almost treating Gatsby like a servant rather than like a host. The woman is nicer, but she is somewhat drunk. This scene though clearly shows that East Egg people, like Tom, Sloane, and the woman, put themselves much higher than West Egg residents like Gatsby. They will use people like Jay - go to his parties, accept his hospitality - but they will not really co-mingle with them.