As of today, Microsoft Planner users can assign multiple people to a task—a feature that tops the list at planner.uservoice.com. Now, users can assign more than just one user to a task in Planner, and every user that is assigned the task will see it on their My Tasks page.
https://blogs.office.com/2017/04/03/assigning-multiple-users-task-now-possible-microsoft-planner/