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 (both bugs & shrimp) into a community pot without tossing in things like 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 make the stock immediately. Also, think about whether you want to rinse the shells to get most of the spice off. If you prefer you can put that decision off until 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
ALTERNATIVE METHOD FOR FINAL FLAVOR STEP: Do not add corn, potatoes or sausage at the beginning. After shellfish is done cooking, instead of the final cooling and 15 minute soak, working quickly turn heat down to low, remove shellfish and layer them in ~2" layers in an ice chest or their shipping container, sprinkling each layer with salt & boil mix (or already salted boil mix). Tightly cover the container and let them steam in their own heat for 15 minutes. While this is happening add the potatoes to the pot and bring to a full boil. Once just fork tender, add any remaining vegetables (corn, mushrooms, etc.) and sausage and cook 5-7 minutes.