From an email from Microsoft:
Invite stakeholders to a project to keep them informed on what’s happening with that team or project. They’ll have access to the team home page, where they can view updated project information. They’ll also have access to the backlog, taskboard, and Kanban board, where they can create and edit work items or work item queries. You can add unlimited stakeholders to your team projects at no charge – in addition to the five Visual Studio Online Basic users you get for free with your account!
If you intend to move existing team member(s) from existing plans to the new stakeholder plan, remember to visit the Microsoft Azure portal and remove any paid user plan that you no longer need, to avoid being charged for them on the next billing cycle.
With organizational accounts, users can use their own company credentials to sign in to Visual Studio Online. To use organizational accounts, link your Visual Studio Online account with Microsoft Azure and set up Azure Active Directory.