Who are NT AUTHORITY\Authenticated Users?
The user “NT AUTHORITY\Authenticated Users” represents every Domain user account that can successfully log on to the domain . In the typical environment that would include employees, contractors, vendors with a “special account” or anyone with Windows Authenticated access to the network.
SharePoint makes it quite easy to add “NT AUTHORITY\authenticated users” to site permission:
CAUTION: You must allow this account permission to your site only when you are convinced that you are willing to reveal your site information to all who can successfully log on to the domain network because the permission you allow to this account would apply to everyone in the domain. Else, ensure if you find this account in your permission list, remove it.
This comes in very handy when you do not wish to add individual permission and you want to make your site at least visible to all or allow elevated permission as and how you plan the entitlements.