Many of you already know Microsoft Planner. It allows you to create a plan, build a team, assign tasks, and update status—all in a few easy steps. Well now Microsoft Planner has added the ability to allow Guest Access and has been enabled across all farms.
Guest access allows you to pull in your outside help and assign them Planner tasks. There of course is some setup involved. Fortunately Brian Smith has a detailed step-by-step post of how to accomplish this. Guess Access in Microsoft Planner