It gets people out of their rut and unites them with the other guests. Choose a drink that can be made ahead of time and in large quantity. After a round or two, fixing individual drinks for a crowd can get tiresome. Offer something equally interesting to those who prefer a non-alcoholic drink. Don't over-water your guests.
Many cocktails are stronger.
Based on the number of guests, make a reasonably-sized pitcher of drinks. When the pitcher's empty, it's time to get dinner on the table. The margarita is the perfect summer drink. It can be made in large quantity very simply, and most everybody loves it. Fill each glass with a portion of drink, straight up or over crushed ice.