Guests, mostly uninvited, go to Gatsby's parties for entertainment, free food and drink, and to some degree, because they have heard such interesting rumors about Gatsby.
Nick notes that they
...came for the party with a simplicity of heart that was its own ticket of admission (45).
One party goer, Lucille, says,
"I like to come....I never care what I do, so I always have a good time.
Lucille also shares that when she tore her gown, Gatsby arranged for a new one to be sent to her.
Following the conversation about the ripped gown, the guests speculate about rumors they have heard about Gatsby, his having been a German spy during the war (World War I) and his having killed a man. Nick says,
It was a testimony to the romantic speculation he inspired that there were whispers about him from those who had found little that it was necessary to whisper about in this world (48).
All in all, these guests are curious freeloaders who are taking advantage of Gatsby's generosity to party freely. It is not so much that they ignore him as it is that most of them do not even know who he is, and he walks through these parties engaging little, if at all, with his guests. His sole purpose is to attract Daisy, and really, he cannot "see" anything else.