You can add a collaborator to your project by simply inviting people via email including people who are not collaborators on GitHub.If the user with that specific email already exists, she will automatically be added to project's collaborators. If the invited user does not exist yet, invitation email will be sent. Once the user follows the link in the invitation email and signs up, one will be automatically added to project's collaborators.
You can remove a collaborator from project just by removing him from project's collaborators list. Once the user has been removed from this list she will no longer have an access to specific project.
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