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).
Select Settings, and then select Site permissions.
Here you can add members and manage permission levels, sharing permissions, and guest expiration.
If you have additional groups or people that you'd like to give permissions to the site, select Advanced permissions settings.