Remember that everyone else is there to meet people, too! Whether the party is lively or more relaxed, you can usually find a quieter place with soft music where you can talk to people. Introduce yourself to someone and start a conversation. Ask what he or she thinks about the music, or how he or she knows other people at the party.
First of all, ask yourself if the person would really like a surprise party. Not everyone enjoys loud music and crowded places! Then decide who you can trust to keep the party secret. It's better if only a few people know about the party in the beginning. Make a list of guests, but don't send out the invitations immediately.