Rewards are at their most effective when they combine an intrinsic and an extrinsic incentive.
If you have a paid (subscription) product, you can harness this knowledge with special group/team pricing as a reward.
For example, you could say "refer five people on your team at work, and you'll all unlock a special team discount on the paid newsletter".
This increases the motivation for the advocate to share, *and* boosts your revenue at the same time. Win-win!