An outside collaborator is a person who isn't explicitly a member of your organization, but who has permissions to a project board in your organization.
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In the upper-right corner of GitHub Enterprise Server, select your profile photo, then click Your organizations.

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Click the name of your organization.
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Under your organization name, click Projects.
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In the projects list, click the name of the project board.
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On the top-right side of the project board, click Menu.
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Click , then click Settings.

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In the left sidebar, click Collaborators.
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Under "Search by username, full name or email address", type the outside collaborator's name, username, or GitHub email.
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In the search results, click the correct username and click Add collaborator.
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Optionally, next to the new collaborator's name, use the drop-down menu and click the desired permission level: Read, Write, or Admin.