Make people choose number (1-30 if you want to limit it into 30 people)
Then do the random stuff ;o
My idea was to just write all the names in the randomizer and when I think I got enough people I will draw the winners. It's kind of easier than to make people choose a number.
You'll need steam for the game anyway, so go make an account. While you're at it you could get Team Fortress 2 for free from the Steam store too, it's quite a good game.