A partygoer is someone at a party. You can't be a partygoer if you don't come to the party.
This word defines itself: a partygoer is someone who goes to a party. The first step to being a partygoer is getting invited to a party. If you show up uninvited, you'd probably be called a party crasher rather than a partygoer. There are partygoers for every kind of party: birthday parties, graduation parties, holiday parties, and work parties. Someone who goes to many parties could be called a frequent partygoer. The more partygoers, the bigger the party.