Organization owners and people with admin permissions for a repository can allow or prevent the forking of a specific private repository owned by your organization.
Note: An organization owner must allow forks of private repositories on the organization level before you can allow or disallow forks for a specific repository. For more information, see "Allowing people to fork private repositories in your organization."
On GitHub Enterprise, navigate to the main page of the repository.
Under your repository name, click Settings.
Under "Features", select Allow forking.