So noting
@SweetieJean 's comment above, and the possibility that you are a credit union, I would add the following reminder.
If you decide this is "other income", then you are only required to issue a 1099-MISC if the value of the reward is more than $600 to any customer in any year. You CAN issue a 1099-MISC if the reward value is less than $600 but you don't have to. If you decide to treat the reward (referrer or referee or both) as account interest, then you would have include it in their annual 1099-INT no matter what the amount is. (I think the threshold for a 1099-INT is $5.)
