You can view and modify user roles in Settings > Users.
One or more people in an organization are also designated to be the Billing Manager(s).
Team vs. Unlisted Projects
Projects are team projects by default, but can also be restricted to be unlisted. Unlisted projects will no longer be visible to anyone who isn't explicitly added to the project.
You can make a project unlisted by clicking the "Restrict Access" button in the project creation and management modal.
You can add Guest Editors or Commenters directly to projects, through the project management modal.
You can invite Team Editors or Commenters the same way, or via the "Users" section of your Settings page.