Create users that can create other users

I am in the process of building a website. It will be a paid subscription site. Need each individual paid user to be able to create a few more users which do not need to pay. The subscribing user sends the email activation link to join using which the other users can join. The subscribing user and the users he joined share some files. The visibility of the files should be restricted to the subscribing user and the users he activated. I am using Sydney theme.
Are there any plugins to achieve this functionality? I have tried ‘Buddy Press’, ‘Groups’ etc but not able to find the functionality I am looking for.
Appreciate any guidance / help.