Former "Take That" star Williams and his wife Ayda Field are friends with Eugenie and her sister Princess Beatrice.
"This is meant to be a family wedding", Prince Andrew said earlier.
British model Cara Delevingne (C) arrives with her brother-in-law James Cook (L) and USA journalist Derek Blasberg (R) to attend the wedding of Britain's Princess Eugenie of York to Jack Brooksbank.
The couple, who had been dating for eight years, Wednesday at the same spectacular Berkshire venue chosen by Prince Harry and Meghan Markle for their May nuptials.
The dress features a low back that shows off her scoliosis scar.
Queen Elizabeth II and her husband Prince Philip attended the wedding, along with Prince Charles, Prince William, Prince Harry and other senior royals.
A royal insider insisted Camilla's absence was not meant as a snub, telling the Mail: "She is fond of Eugenie and her sister Beatrice, and is very disappointed that she can not be there".
It was reported guests had been asked to "surrender mobile telephones, cameras and any electronic devices used for image capture" for the afternoon reception at Windsor Castle, hosted by the Queen. She works in the art world.
The couple got engaged in January when Brooksbank, 32, proposed during a holiday trip to Nicaragua.
The bride's parents left the chapel together smiling as the newlyweds embarked on a horse-drawn carriage ride through parts of Windsor.
Several hundred singing and cheering well-wishers gathered outside in Windsor in the shadow of the castle, far fewer than the thousands that crammed into the town for Harry's wedding.
The ceremony was overseen by the Dean of Windsor David Conner and charity guests and 1,200 members of the public were invited into the grounds for the occasion.
"I'm a royal superfan, so when her majesty organises a big event for her granddaughter, I can't stay at home", said Joseph Afrane, 54.