You can 'recruit' players who are on the free trial. So perhaps reach out to trial players on online forums, or even in game if you happen across them.
Both you and your recruitee will get rewards out of it so it's worth their while. They just need to enter your recruitment code when they make their first payment/subscription.
It just relies on making new friends who happen to be on the trial and close to subscribing, which isn't exactly easy to find but they're out there.