Many products sold on online platforms have additional features. Platforms can deliberately shroud these features from consumers, e.g. by revealing them only late in the purchase process. For example, platforms often reveal shipping- and handling fees, or upgrades like luggage or hotel services only late in the purchase process. We study when a two-sided platform discloses (a.k.a unshrouds) additional fees when some buyers naively ignore shrouded fees. We uncover a novel mechanism to explain why platforms shroud: platforms shroud or unshroud to manipulate cross-group externalities between buyers and sellers. Exploring this mechanism, we highlight two results suggesting online marketplaces lead to more shrouded features. First, we ask when a platform shrouds seller fees on its marketplace. Driven by cross-group network externalities to attract buyers and appear cheap, the platform has stronger incentives to shroud seller fees than sellers themselves. Second, we investigate when the platform shrouds its own additional fees and uncover a perverse effect of seller competition: fiercer competition between sellers encourages the platform to shroud its own fees. Both results hold even though the platform earns no commission to shroud seller fees and does not sell its own brands, so banning these practices will not induce a transparent marketplace. We discuss further policy implications and connect to common practices like drip pricing, steering, and rebate design.