What determines what user / groups Ranger can see when setting policies?

Have users on local machines that have HDFS /user dirs that do not show up as possible users when setting Ranger policies I can see that Ranger already have a place where you can see and add users in the settings menu of the ranger UI, but not sure where this is getting populated from. So my question then is what determines if Ranger can see cluster users for setting policies (and is there an easy way to manage this via ambari)?

The problem was that I had thought, looking at a answer on the Hortonworks community forums, that for a user to be recognized as "existing" on the HDP cluster, all that was required was for the user to 1) exist on a cluster node and 2) have a folder in hdfs:///user/<the username>. This apparantly is not correct (at least in the case of being recognized by Ranger as a valid user that can have policies set on them).

In order for a user to be recognized by Ranger (here, I do not have a cluster integrated with Kerberos or Active Directory), that user needs to exist on the usersync server machine which supports...

the ability [for Ranger] to get users and groups from the corporate AD to use in policy definitions.