From my experience with other referrals, app based referrals can be much harder to track than URL based ones (American Express as an example) since the app is downloaded through the App Store and then signing up, vs. completing a card application in the same session the link is clicked.
I’d suggest that the referral process involves entering an email and/or phone number to invite the user via a personalized message with a download link. URL based tracking would be available through the message sent to them on the back end (similar to right now) and then the email address / phone number of the new member would link back to the original member who referred them. I think part of the success of a referral program will depend on the robustness of the program, both the existing member and new member would quickly become discouraged if bonuses didn’t work properly.
I’m sure it can be improved, but may involve a reworking of the back end. I think this also offers an opportunity for the referral program to be reworked, maybe offering new users a trial of the benefits of Black during their first month and rewarding the person who did the referral with free months of service or credits like in the current model. That would offer some incentive to sign-up via referral, incentive to refer friends and hopefully help Curve convert more users into Black or Metal users.
There’s been some discussion previously, found here, which might be of interest to you.