Sub-User Creation

I am basically trying to create a main user and sub-users under them. When a new user is created by the admin, he can have the option of nesting / associating the user under a main user and in the front-end a filter is required to display the main users and his list of sub-users. Sub-users also needs to be created under admin.

I have looked around but could not find a solution to this. Can someone please help me on this. I really need it.