In canon appearances
The Rescue Team has united (though on smaller scale) in such games as Super Mario 64 DS, New Super Mario Bros. Wii, and Super Mario Galaxy. They also more or less unite in Mario spinoffs.
In fanon, Booster has been added to the team. They are most prevalent in the Koopa Family Adventure games and the Bowser and the Koopalings games. Often, they serve as major bosses and obstacles to Bowser.
Mario, though it is not explicitly stated, is the leader of the team and Princess Peach's right-hand man. He is often the final boss.