you basically ask people to bring a wrapped ornament, preferably around the same price point, maybe $5. label all the ornaments with a number, then have the people that brought an ornament pick a number to receive a new ornament.
this is the sign up sheet; you can cut it in half. before your holiday party have people sign up on the left side so you can gauge how many people will participate. on the day of the party, used the numbered side on the right to keep track of the number people pick for an ornament.
this is the number sheet for corresponding the ornaments to their new owners. there's two sets of numbers: one goes on the wrapped ornament, the other is for people to draw their number. print it on a sticker sheet to turn them into stickers, or print on regular paper and tape the numbers on.
have fun at your holiday parties!