An organization owner can restrict the ability to invite collaborators. For more information, see "Setting permissions for adding outside collaborators."
Before you can add someone as an outside collaborator on a repository, the person must have a user account on your GitHub Enterprise Server instance. If your enterprise uses an external authentication system such as SAML or LDAP, the person you want to add must sign in through that system to create an account. If the person does not have access to the authentication system and built-in authentication is enabled for your enterprise, a site admin can create a user account for the person. For more information, see "Using built-in authentication."
If your organization requires members and outside collaborators to use two-factor authentication, they must enable two-factor authentication before they can accept your invitation to collaborate on an organization repository.
When you add an outside collaborator to a repository, you'll also need to add them to any forks of the repository you'd like them to access.
On your GitHub Enterprise Server instance, navigate to the main page of the repository.
Under your repository name, click Settings.
In the left sidebar, click Collaborators & teams.
Under "Collaborators", type the name of the person you'd like to give access to the repository, then click Add collaborator.
Next to the new collaborator's name, choose the appropriate permission level: Write, Read, or Admin.