About team maintainers
People with the team maintainer role can manage team membership and settings.
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Remove the team's access to repositories
 
Promoting an organization member to team maintainer
Before you can promote an organization member to team maintainer, the person must already be a member of the team.
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In the upper-right corner of GitHub, click your profile picture, then click Organizations.
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Click the name of your organization.
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Under your organization name, click Teams.

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Click the name of the team.
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At the top of the team page, click Members.

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Select the person or people you'd like to promote to team maintainer.

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Above the list of team members, use the drop-down menu and click Change role....

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Select a new role, then click Change role.