When I signed up yesterday, the referral code field was greeted out. I noticed a code below the referral field that looked like referral code, with an 'x' next to it. I tapped on the 'x' which made the code disappear, the referral field was no longer greyed-out and I was able to fill in another member's referral code.
It might be too late if you've already registered, but hopefully this helps someone going forward.
