First of all, it's Booster, not Wario.
Here's what you do. Get Bowser to leave, then check the door to find that it's locked. Try to leave the area and Bowser will come back, bust the door open, and sign you up to the Koopa Troop ;). He really just joins your party after he breaks the door down.
Mario, Mario, Mario! What happened to Mr. Andretti?