Potluck! :)

Is it going to be cookout type food or is it an indoor party?

You could have a large salad or a fruit salad. Both are easy and no cooking is required.

I found 2 recipes that seem pretty simple and are low in calories:

Broccoli-Bacon salad: http://www.delish.com/recipefinder/broccoli-bacon-salad-recipe-6461

Seven Layered Salad: http://www.delish.com/recipefinder/seven-layer-salad-recipe-6851

If you have a crockpot you can try this 3 Bean Turkey Chili: http://www.skinnytaste.com/2009/01/crock-pot-3-bean-turkey-chili-3125-pts.html

Also in a crock pot, you can make buffalo chicken and bring some finger rolls or something similar if people want to make little sandwiches. http://www.myfitnesspal.com/topics/show/621282-crock-pot-buffalo-chicken-and-very-low-calorie