Almost every day when they get home from school, Gracie, age 16, and Sarah, age 14, open the app Houseparty, where they can video chat with to up to seven of their friends at once. The sisters, who live in Danville, Calif., use it to socialize and collaborate on homework, for 15 minutes to an hour. When they first open it they may be chatting with just one friend, but everyone they’re connected to on Houseparty gets a push alert that they’re “in the house,” and, soon enough, the room fills up. It might even spill over into other rooms, growing organically, just like a real house party.

Teens...