I've had a few, but none that lasted more than a month or so, until I wrote a magazine article that explained what Medium is and invited people to join using my referral link. That got a few more that have stuck about for longer. It's not been a big winner though, and in all honestly, I think the early ones are more likely that people read my article then joined, rather than joined using my referral link. I've heard that if someone reads an article then joins, the author gets that referral automatically.