Check with Paid Memberships Pro support + see if their code supports this.

You may have to write a light weight custom WordPress plugin to automatically create custom roles for your clients + sub members.

You'd do this by "hooking" the PMP create event... actually the event which passes the MemberOrder object.

https://www.paidmembershipspro.com/hook/pmpro_after_order_settings is the likely hook.

You'd have two membership levels. One for clients + one for sub-members.

When pmpro_after_order_settings fires for new sub-member subscription, then code would associate subscribing sub-member to client.

Likely best you hire someone familiar with setting up + hosting complex WordPress Membership sites.

Your site design must reflect both the functionality you require + be coded in such a way as to require very little run time resources.

I've had to redesign many client's sites, when I take them on as a private hosting client, because the developers who wrote their code had no clue about writing code to run fast, in situations where there are many simultaneously logged in users... or complex data must be searched...

Best to start with a stellar code designer.