This is what I have in mind:
Customer A has a basic subscription. We give her a referral link like "domain.com/subscribe.php?refid=customer_A" that she can send to her friends. If X friends purchase a basic subscription through that link, customer A gets 1 month of premium subscription for free. After that month, she goes back (automatically) to the basic subscription.
Even better:
Customer A has a basic subscription. We give her the same referral link as above. If X friends purchase a basic subscription through that link *and* each of them keeps that subscription for 3 months, customer A gets 3 months of premium subscription for free. After 3 months, she goes back (automatically) to the basic subscription.
Bernardo.