Spooky Parties: How do I get Sims to come in the costumes I set for them?

The spooky party will cause guests to arrive in a random costume that was available at the time the Spooky Stuff pack came out. If your costume choice is the only option in the "Party" category then Amandele's suggestion will suffice. But you can't choose which party outfit so if they have more than one set they may or may not choose the costume.