What is a referral bonus?

How is it achieved?

Is it worth the trouble trying to get people to use me as a referral?

How many people does it take to earn the 5th rank?

Referral bonus is points you receive when someone you referred reach a certain goal in battling, nurturing, or breeding. When they hit that amount, you'll get 1 point each, so from each person you refer, you can get up to 3 points.

These points can be used to increase gold/experience from battling, amount of scales you get from nurturing, or decrease the time it takes for your eggs to hatch.

You will need at least 2 people to reach the 5th rank for one type of reward. More, if the 2 you referred did not hit the goal for the 3 types or experience.

Whether it's worth the trouble depends on how much you want the rewards. If you've filled up the 3 ranks, ie have 15 points, any referral points after that will be converted to diamonds. I'm not sure how much diamonds the reward is, since I have not reached that stage.