Why for free? Give, like, a 50% discount. Their "free stuff" would be "50% for free".

Or even 90%. Maybe 75% as a compromise. Heck, being grandfathered into a very low price point will make them much less likely to quit even if they no longer use the product, thanks to FOMO. So you may see long-tail income from those early adopters with little expense.

Or a 5%-15% forever discount per referral that's paid for a year. After 6-20 referrals, you're on us - it's cool.