Open Microsoft Planner with Microsoft Office 365 credentials. 

Click on the new plan the pop-up will come up which will ask you for a plan name and also give you the option of to create a public plan or private plan. 

So the public plan has access to all the people inside the organization so everyone in the organization can access your public plan secondly the private plan so all the only the members who is added to the particular plan can only see the activities or the contents of the private plan