On top of the diamonds you get for each individual friend who joins and plays, Chillverse also rewards bigger milestones as your total referral count climbs. Reaching 5 successful referrals earns you a 20-diamond bonus, 10 referrals earns another 40, 15 referrals earns another 60, and 20 referrals earns a final 80 — for a maximum total of 200 bonus diamonds across all four milestones once you've referred 20 friends.
These milestone rewards are separate from (and on top of) the per-friend bonus you already get, and your progress toward each milestone is shown right on the Referral page so you always know how close you are to the next one.