You will want a look that goes with the type of summer party in question. If you are meeting friends at the beach, light or bright colors and water-friendly clothes are de rigueur. A bathing suit topped with an easy to slip off beachwear dress, plus flat sandals, would be great.
For a party in the country, a floral dress or pastel shades are a good choice. You don't want to look like you are attending a funeral, so no all black outfits, please. Also, keep in mind that high heels are not very lawn-friendly. Flats or wider heels are much easier to walk in outdoors.
For hot, sunny weather, a good hat can be a good addition to your outfit. It can provide both personality and practical protection from the sun. It is also a great way to hide disheveled, damp hair quickly and stylishly.
If there will be some time spent on the water, weather beach, pool, river or lake, and some time spent in a more formal party environment, a creative approach can help make things wonderfully convenient and simple. Consider adding a sarong to your bathing suit to convert it to a dress appropriate for socializing.