If you simply can't cook for everyone, don't hesitate to turn the party into a potluck. To avoid a buffet of repeated dishes, assign something to each person according to the first letter of their first name. For example: salad for those whose first names start with A to E, chips and dip for E to J, side dishes for the remaining letters, etc. If you can cover the main dish (meats/poultry/fish for grilling out), getting people to bring the odds and ends shouldn't be a problem.