Do not add oil, butter or any form of fat the boil. It steals some of the oil-soluble flavors, and makes cleanup more annoying.
If you can convince your guests to put the shells into a community pot without tossing in cigarette butts or soiled napkins, you can use them to make an amazing stock. They spoil quickly so freeze them right away if you aren't going to get right to making the stock. Also, think about whether you want to rinse the shells to get most of the spice off, and whether you want to do that before you freeze or after you thaw. I like to leave it on, but it may not be appropriate based on your intended use for the stock.
Hardware: Paddle, Big Pot
Final product: Crawfish Stock, Po Boys, Etouffe