The dress code "Dark suit" which is frequently used in Norway only indicates the man's outfit, but there is of course a similar description for women.
For men, it's easy, a dark suit in either black, blue or gray is the way to go. Shirt and tie belong, and you should wear nice suit shoes with matching belts.
For women, you can choose between a long and knee-length dress. An important rule at weddings is that no woman, except the bride, wears an all-white outfit. It is still acceptable for female guests to wear a partially white outfit, as long as the white is not the dominant color of the outfit. Red, strong colors and thigh-length dresses should preferably not be used.