You can organize parties once you have unlocked the Luxury Hotel in Las Vegas.
Upon entering the main hall, go into the suite and speak with the receptionist, rent the apartment and talk to the Butler inside to start organizing your party.
If Las Vegas is still locked, just keep completing story quests, shoot movies, work in the Harlem Restaurant, search for collectibles or date somebody in the karaoke bar so you can gain experience, progress in the story and level up. Las Vegas gets unlocked once you reached level 14.
Upon entering the main hall, go into the suite and speak with the receptionist, rent the apartment and talk to the Butler inside to start organizing your party.
If Las Vegas is still locked, just keep completing story quests, shoot movies, work in the Harlem Restaurant, search for collectibles or date somebody in the karaoke bar so you can gain experience, progress in the story and level up. Las Vegas gets unlocked once you reached level 14.