From DrinkiWiki

To Start the Game:

Each player chooses either to play as Alex, Marty, Melman or Gloria. (If your name is Alex, Marty, Melman or Gloria you don't get a choice you HAVE to play as the character who shares your name.)

The Rules:

If you chose Alex, Marty or Melman: Take one shot of beer or a mixed drink each time your character's name is said.

If you chose Gloria: Take a shot of hard alcohol every time her name is said.