Registered user can register another user

Heya, folks!

Just wondering if anyone had an idea how to make this scenario work:

"User A" subscribes to my site and gets assigned under "Level 1".

I want to then allow this user to set up / add multiple registrations / subscriptions for "User B", "User C", "User D" etc. for which these new users get assigned under "Level 2".

If there is a straight line function where I can insert the users information directly into the database tables, that would probably be the best way... just don't know which tables are involved / touched upon a new registration.

I can build the form and function myself, just need to know what pieces go where in the database I guess.

Thanks much!