You have to have a referral to join so everyone can use mine. Obviously it benefits me a little bit if people use my referral, but you can probably find other referrals easily enough if you want.
Pi is a new digital currency developed by Stanford PhDs, with over 10 million members worldwide. To claim your Pi, follow this link https://minepi.com/QNorth and use my username (QNorth) as your invitation code.