From the Super Mario Wiki, the Mario encyclopedia
The Bob-omb Mafia, also known as Bob-omb crowd, Bob-omb club or Bob-omb gang, is a large group of Bob-ombs which Big Bob-omb leads. They make their only appearance in Mario Party Advance. While traveling through Shroom City, the player encounters several people who mention that the mysterious leader of the Bob-omb Mafia is located in a secret room somewhere in Shroom City. Going to a hidden floor in the Mushroom Condo, the player finds Big Bob-omb and challenges him to a game of chance. Earning the respect of Big Bob-omb, the player gains a minigame and Gaddget.