A collaborator is a person who has permissions to a project (classic) you own. A collaborator's permissions will default to read access. For more information, see "Permission levels for a project (classic) owned by a personal account."
Inviting collaborators to a user-owned project (classic)
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Navigate to the project (classic) where you want to add an collaborator. 
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On the top-right side of the project (classic), 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 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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The new collaborator has read permissions by default. Optionally, next to the new collaborator's name, use the drop-down menu and choose a different permission level. 
Removing a collaborator from a user-owned project (classic)
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On the top-right side of the project (classic), click Menu. 
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Click , then click Settings.  
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In the left sidebar, click Collaborators. 
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Next to the collaborator you want to remove, click X.