An outside collaborator is a person who is not a member of your organization, but has access to one or more of your organization's repositories. You can choose the level of access to grant for each outside collaborator. 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. If you are adding an outside collaborator to a private or internal fork of a repository, the collaborator must be a collaborator on the upstream repository.
Unless you are on a free plan, adding an outside collaborator to a private repository will use one of your paid licenses. For more information, see "About per-user pricing."
Organizations that use GitHub Enterprise Cloud can restrict the ability to invite collaborators. For more information, see "Setting permissions for adding outside collaborators" in the GitHub Enterprise Cloud documentation.
If your organization requires two-factor authentication, all outside collaborators must enable two-factor authentication before accepting your invitation to collaborate on a repository. For more information, see "Requiring two-factor authentication in your organization."
You can give outside collaborators access to a repository in your repository settings. For more information, see "Managing teams and people with access to your repository."