TechStyle Fashion Group, the company behind Kate Hudson’s Fabletics; the children’s brand FabKids; Rihanna’s lingerie brand, Savage X Fenty; and the shoe retailer Shoedazzle, has amassed 5 million members for its discount retail program. Those members were a large driver behind the $750 million in transactions the company saw for 2018, according to co-chief executive Adam Goldenberg.