Drupal 7: Find which user created a given user I have a number of users admins (group: "users_admin") who create users who can view content (in many groups, like "students_a", "students_b", "students_c", and so on).  

Is there a way to filter the users by their creators?  

Let me clarify:  


User "A" from "users_admin" create a user "S1" in "students_a".  
User "B" from "users_admin" creates a user "S2" in "students_a".


I would like to be able to differentiate users created by "A" and "B", has anyone done that before? Is it possible?  

I hope I made myself clear enough; please tell me if you need some clarifications.  

Thanks in advance for you insights!
Regards
Drupal, by default, does not keep any authoring information of users. 

However, there is a module for that! Try Sub User module. It uses Relation module to maintain the relationship with parent user.