Play As Bosses:
To play as the bosses, choose team play & pick yes when asked if you want to edit the team. Hold Start & press Up + Circle, Right + Square, Left + X, Down + Triangle. If done right, the bosses will appear.

Select Team Play:
At the character select screen, Hold Start & press Up + Circle, Down + Triangle, Left + X, Right + Square, Up + Circle. You should hear a tone. Now you can select 2 or 3 of the same character for the one team.