The answer to this question depends on which party you are referring to, and that depends a lot on what the definition of a "party" is. Here are some possibilities:
- Party at Daisy's House (chapter 1). Daisy, Jordan, Nick, and Tom are there, so it might count as a party. If so, Nick meets Jordan and Tom for the first time.
- Party at Tom's "love nest" (chapter 2). Tom, Nick, Myrtle, Catherine, Mr. and Mrs. McKee. At this party in the hotel Nick meets Catherine, Mr. McKee and Mrs. Mckee for the first time. Technically he met Myrtle before the party started.
- Party at Gatsby's (chapter 3). At this party, Nick meets Gatsby's chauffeur (though technically he meets him before the party. He also meets a pair of unnamed twins and a guy known as "owl eyes" who is drunk in Gatsby's library.
- Party at Gatsby's (chapter 4). At this Sunday party Nick meets quite a few guests, though they are all unnamed minor characters that aren't important to the overall story.
These are all possibilities when it comes to answering your question. My suspicion would be that #3 is the one you're looking for, as that's the best documented of the parties that involves a lot of guests.