Multi-Refer Bonus Reward 3 - Bright Engram. I mean, it's really hard to get there why a bright engram as a reward. it should be better. Unless its a lot of engrams which i doubt.
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It is rewarded at 4 total refers and every time you Refer a Friend and complete the required steps following that fourth time. It is to give some incentive to continuing to Refer after you have done the 3 required to get the sparrow and the ornament.