In Office SharePoint Server 2007 and the related technology SharePoint Services 3.0 we were able to add “All Authenticated Users” by simply clicking a link on the “Add user to group”-page. SharePoint 2010 does not have this link, so we have to enter the groupname into the field. Have a look into the article “What happened to Add All Authenticated Users?“. Here you will find a description.
If you use a German version, you have to enter “NT-AUTORITÄT\Authentifizierte Benutzer”.