Steal Coins
Pay Boo five coins and he will take a random amount of coins from another player. Coins are needed to buy bonus items that will help you succeed on each board. For example, purchase a Golden Mushroom for 20 coins to roll three dice blocks instead of one. This can help you move around the board faster.
Defending Against Boo
Another player could send Boo over to steal some of your coins. Hit the "A" button rapidly to repel the ghost and reduce the number of coins he is able to steal from you.
Take a Star
If you manage to collect 50 coins you can pay Boo to steal a star from another character. This will greatly increase your chances of winning, but amassing 50 coins is no easy feat. You earn coins by landing on certain-colored spaces on the board and by winning the mini-games. Save as many coins as you can so you can summon Boo when you need him.