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The Prince was joined at BBQ restaurant Rendezvous by his brother Prince William and their cousin Princess Eugenie as well as the bride and groom-to-be.
Harry was dressed down in jeans and a light blue shirt while William opted for a red and blue checked shirt and jeans.
American-born bride-to-be Lizzy Wilson looked effortlessly elegant in a beige patterned dress, a white blazer and silver heels, her glittering engagement ring prominent on her left hand.
Having heard that the party were in town, locals in the Tennessee city gathered outside the restaurant to catch a glimpse of the Princes and their high-society pals.
Harry arrived in the US on Wednesday in the wake of news that he has split form his girlfriend Cressida Bonas. Leaving his relationship troubles behind him, Harry flew to Miami for Guy's bachelor party.
The newly single Prince, who dated Cressida for two years, was seen celebrating the lead-up to Guy's wedding, which will take place on Saturday 3 May.
Lizzy Wilson and Guy Pelly
The three-day wedding celebrations will kick off on Friday with drinks at the Memphis Hunt and Polo Club, close to Lizzy's family home.
Guests will also have the opportunity to visit Elvis Presley's mansion nearby, where the King of Rock lived and passed away.
The main ceremony will take place on Saturday, after which guests are expected to attend the reception in fancy dress costume and party the night away. Sunday will see the extravaganza end with a lavish brunch at the club.