There is no easy way to do this.
You could hook into whatever event you are interested in rewarding, then use get_user_meta to retrieve the referrer id. Finally, use the move action to transfer coins from one account to another.
If you are going to reward users for logging in you must rate-limit this somehow, otherwise your users will be logging in constantly. I would store the last login timestamp in another user meta and check if some time has elapsed.
kind regards