You can set base permissions that apply to all members of an organization when accessing any of the organization's repositories. Base permissions do not apply to outside collaborators.
By default, members of an organization will have Read permissions to the organization's repositories.
If someone with admin access to an organization's repository grants a member a higher level of access for the repository, the higher level of access overrides the base permission.
Note: All changes to base permissions will affect both new and existing members.
In the upper-right corner of GitHub.com, select your profile photo, then click Your organizations.
Next to the organization, click Settings.
In the "Access" section of the sidebar, click Member privileges.
Under "Base permissions", select the dropdown menu and click a permissions level.
Review the changes. To confirm, click Change default permission to PERMISSION.