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.
Our goal is to support additional collaboration, and we will continue to develop features and enhancements that our users want. Feel free to join the conversation about this feature and many others at our TechCommunity page. Also, please share your feedback with us about Planner features you would like to see at planner.uservoice.com.
—The Planner team
Source: Microsoft Office Blogs