Monthly (or quarterly) contest. User with the most referral signups gets the holy grail within Zwift… a custom kit (maybe even bike, helmet, etc if you really want to go big).
referral programs are a delicate beast. No need to impact margins… gamify a referral program to an already competitive group seems like a no brainer in Zwift context. This is a pretty awesome reward IMO.
For a good reference go checkout hotjar’s referral program. The UI is slick, and is a realtime scoreboard. With the talent you have… maybe 2 days worth of coding. 1 month beta test as the trial… off you go.