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.