Do not add any butter or oil to the boil. It hogs 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 any 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, probably best to rinse the shells well to get most of the spice off, though leaving it all on would be an interesting experiment.
The quick-cool down is important. There is a nifty (and spendy) tool called the "Boil Boss" which can be a big help. DIY types can put one of their own together easily for about $10.
Hardware: Paddle, Big Pot
Final product: Crawfish Stock, Po Boys, Etouffe