Among the first of the couple's guests to arrive in London was "Suits" co-star Gabriel Macht, who plays Harvey Specter.
The 46-year-old American actor and his actress wife, Jacinda Barrett, have been taking in the sights around the British capital, posting pictures on their Instagram accounts.
In one, Barrett is perched outside Buckingham Palace receiving a kiss from her four-year-old son. Barrett also starred on "Suits" as Specter's one-time love interest, Zoe Lawford, between 2012 and 2013.
Patrick J. Adams, who portrays Meghan's onscreen husband Mike Ross, revealed on Thursday he had landed in the country and gone straight to the pub.
Updating his 1.9 million Instagram followers, the Canadian actor uploaded a snap from the Royal Standard of England -- a pub in Buckinghamshire that claims to be the oldest in Britain.
He later added a photo of himself outside the watering hole alongside his wife and fellow actor Troian Bellisario.
Their co-star Sarah Rafferty, who plays executive assistant Donna Paulsen, has also touched down in London after a quick pit stop in Paris with her family.
Rick Hoffman, who plays Louis Litt in the show, posted a video on Instagram telling fans he was "excited" to be "heading east for some special event."
Other former co-stars and close friends of Meghan, including Abigail Spencer, Gina Torres and Amanda Schull, are also expected to attend.
Another friend, Misha Nonoo -- Harry and Meghan's rumored matchmaker -- also jetted back into London earlier this week. The Bahrain-born, London-raised fashion designer confirmed she was back in the city in a black-and-white shot of her walking down a London street.
Meghan's mother, Doria Ragland, arrived in London on Wednesday. Ragland was reportedly met on the tarmac by Meghan's assistant Amy Pickerill and whisked off to be reunited with her daughter.
Ragland had tea with Harry's father, Charles, the Prince of Wales, and his wife, the Duchess of Cornwall, at Clarence House in London on Thursday, Kensington palace said. She later met William and Catherine, the Duke and Duchess of Cambridge, and their two older children, Prince George and Princess Charloote, during wedding rehearsals in Windsor.
Ragland is due to meet Queen Elizabeth for tea at Windsor on Friday before spending the night with her daughter at the luxury five-star Cliveden House Hotel.
One confirmed no-show will be Meghan's father, Thomas Markle, who has had to pull out over health problems.
"Sadly, my father will not be attending our wedding. I have always cared for my father and hope he can be given the space he needs to focus on his health," Meghan Markle said Thursday in a statement released by Kensington Palace.
Thomas Markle, who has been the subject of a controversy over staged photos in recent days, was due to walk his daughter down the aisle.
Prince Charles will now accompany Meghan for part of her journey to the altar on Saturday, Kensington Palace announced Friday.