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

Thank you everyone for your help. :) I managed to get my hosts to change their costumes by sticking a wardrobe on the lot. Couldn't do anything about the guests but on this try the costume choices were much better so I went with that.
I actually quite like the idea that you never know who's going turn up in what. Will have to experiment with this too...