It's an uncredited appearance, lasting just a second or two, during Bill and Fleur's wedding near the beginning of the film — but he's there.
Watch it for yourself — from 3:59 to 4:03 in the clip below.
That Harry Potter appearance wasn't the only role Regé-Jean had before Bridgerton, of course. He also starred in the 2016 remake of Roots, as well as 2018's sci-fi action film Mortal Engines.
More recently, he was also in Sylvie's Love, which is currently available to watch on Amazon Prime Video.