About base permissions for projects
You can set a project's base permission to control the level of access for all members of your organization. You can then specify individual and team permissions for each project in addition to the base permission. For more information on setting permissions for individual projects, see "Managing access to your projects."
When you set the base permission for projects in your organization's settings, the base permission will apply to any new projects created by organization members and existing projects that do not currently have a base permission configured in the project's settings.
Setting a base permission for projects in your organization
In the top right corner of GitHub.com, click your profile photo, then click Your organizations.
Next to the organization, click Settings.
In the "Access" section of the sidebar, click Member privileges.
Under "Projects base permissions", select the dropdown menu and click a permissions level.
In the pop-up window, review the changes and number of projects that will be affected. To confirm, click Change default permission to PERMISSION.