The organization owners can add or remove members in organizations.
Adding members to organization doesn't mean that they have access to all the projects inside that organization. You should add each member to projects. You don't need to enter their email address after you've added them to organization.
Please notice that if you delete an organization, all the projects it contains will turn to personal projects.
For removing a project from an organization you can go to "Project Settings" and change the organization to "Personal".
If your need to have more than 10 members in an organization, you should Upgrade Organization to Business Plan.