This is a multiplayer stadium. Play with your friends either 1v1, 2v2, 3v3, or 4v4.
EVERYBODY needs Bakkesmod Net Code Plugin (#166) in order to BOOM.
Press the "Use Item" button to BOOM. BOOM next to the ball. BOOM next to other players.
Recommended game settings:
Respawn Time - 1 second
Also try boomer mode settings for more fun.
My personal favorite was heavy ball.