From time to time, Chillverse shows a short popup encouraging you to invite friends through the Referral program, with a reminder of the diamonds you and a friend can both earn. It only shows up if you've never actually visited the Referral page, and it stops for good the first time you do — so once you've checked out Referral even once, you won't see it again.
If you ever spot a promo popup that seems out of place — for example, one that doesn't match something you've already done or unlocked — let Support know with a screenshot so it can be looked into.