When I am adding a user to a project I get a yellow note:
NOTE: The user xxxxxxx@outlook.com has access to limited functionality as a Stakeholder. Basic access level is required for access to version control and Agile tools.
Also the user sees a message
As a stakeholder, you can access the backlog, task and Kanban boards, and work items. Learn more.
And the user doesn't have Code and Build tabs
I can add the same user to another project and it works fine.
I had impression that Microsoft was offering 5 free user for Visual Studio Team Services/visualstudio.com.