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.
Adding an outside collaborator to a private repository will use one of your paid licenses. For more information, see "About billing for your enterprise."
An organization owner can restrict the ability to invite collaborators. For more information, see "Setting permissions for adding outside collaborators."
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."