In Planner, select Add Members, and then enter the name of the person you want to add.
Note: Want to add people outside of your organization? This is in development and not yet available in Planner. For more information on features currently in development, please see the Office 365 Roadmap.
After you've added people to your plan, you can assign them to tasks.
Need to remove someone?
If you no longer need someone on your plan, select the arrow next to the plan members, point to the person you're removing, select the three dots, and then choose Remove from plan.