It's a good idea, and one that I've run during the closed beta. A handful of the users have a referral credit for referring other beta users, and once we leave the closed beta, their credit will go into effect. I plan on extending the referral program after the launch as well.

GP has already gotten a lot of value out of them, so if keeping them isn't costing much, I'd say grandfather them in for free.

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.