To redeem a referral code, just tap the link you received (should look like greg.app/join/yourfriend) and create an account! Your 3 free months of water reminders will be automatically applied to your account.
You may need to restart the app, but you'll see a Super Greg flair around your avatar in the Settings sidebar and will also receive a confirmation email letting you know how long your promo is active (check your spam!).
If your promotion is not applied, email support@greg.app with your username and we'll make sure you're taken care of!