A palace source confirms to PEOPLE that on Thursday the invitations began leaving the office of the Lord Chamberlain – the man entrusted with seeing the cards get to the right people.
"They're in the process of being sent out," says the source.
As the deliveries started to go into effect, a charity bash in London was drawing a crowd from the wedding party's circle, including best-man-to-be Prince Harry's on-off girlfriend Chelsy Davy.
Also making the scene at royal pal Guy Pelly's new club Public: William and Harry's cousins Beatrice and Eugenie, along with the bride-to-be's sister Pippa (just named maid of honor), newly engaged Holly Branson and some of William's closest friends like Tom van Straubenzee and his brother Charlie.
"They all looked happy and excited at the prospect of the evening and the coming months," a bystander tells PEOPLE.
As the Duchess of York, in her chauffeur-driven Bentley, dropped off daughter Eugenie, 20, she told onlookers that she was happy to leave the revelry to the younger set.