This isn't an Australian exchange so I'm assuming the $20 bonus and minimum required trade is in US dollars.
You receive your referral reward when the person you invited makes a trade for $100 or more within 30 days. Each successful referral earns both you and the person you referred a $20 cash bonus. A referral earning cap of US$500 per calendar year applies.
Will you be able to get your funds out though? Old timers in the cryptocurrency space might smile on the recalling this image.