By default, new organizations are configured to disallow the forking of private repositories.
If you allow forking of private repositories at the organization level, you can also configure the ability to fork a specific private repository. For more information, see "Managing the forking policy for your repository."
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In the top right corner of GitHub.com, click your profile photo, then click Your organizations.
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Next to the organization, click Settings.
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In the "Access" section of the sidebar, click Member privileges.
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Under "Repository forking", select Allow forking of private repositories.
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Click Save.