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Site permission levels are based on SharePoint permission groups. Members of a SharePoint permission group can either visit the site without editing it (Site visitors), edit the site (Site members), or have full control of the site (Site owners).
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Select Settings, and then select Site permissions.
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Here you can add members and manage permission levels, sharing permissions, and guest expiration.
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If you have additional groups or people that you'd like to give permissions to the site, select Advanced permissions settings.