If you find Luigi five times while playing through the game, he'll appear at the start of the end credits, holding the banner. The locations where you can find Luigi are as follows.

Location	Description
Drybake Stadium	Near the top of the tower on the outside, in the window.
Goomba Fortress	At the second bridge in the middle of the level.
Rugged Road	In the spa area.
Whitecap Beach	At the very end of the dock you can see him in the distance.
Whiteout Valley	Visible as you are going up the ski lift.