These aren't the only Royal guests, if you want to see all of them and what they wore. Check this link. These aren't in alphabetical order.
wind-blown hat, and is wearing the Dame Grand Cross of the Royal Victorian Order. Princess Letizia wore a gray lace dress, a gray floral hat, and the Dame Grand Cross of the Order of Charles III. Crown Princess Mary of Denmark wore a white dress with floral prints, a blue veil/floral hat, and the Order of the Elephant.
Order of the Precious Crown. Princess Mathilde, Duchess of Brabant wore a pink dress with long sleeves, a large pink hat, and the Order of Orange-Nassau.
Order of Adolphe of Nassau. Crown Princess Victoria wore a beige dress with a giant hat, which included flowers and feathers; and the Royal Order of the Seraphim
Order of Merit of the Principality of Liechtenstein. Princess Aimee wore a gray/green maternity dress (she's expecting for the 3rd time!) and a orange hat Maxima has worn. Princess Irene wore a white lace dress,a matching hat and the Order of the Netherlands Lion.
Princess Laurentien wore a red hat, a red outfit and the Order of the House of Orange. Princess Marliene wore a pink dress and a pink hat to match.