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 upper-right corner of GitHub, click your profile picture, then click Organizations.
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Select an organization by clicking on it.
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Under your organization name, click Settings. If you cannot see the "Settings" tab, select the dropdown menu, then 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.