I just committed to the 4.7 version of User points, a new contributed module called referral_points.
The module was written by Peter Lindstorm of Liquid CMS, with some modifications by me.
What it does is intercept all points earned, and awarding the referring user an admin defined percentage of the points the referee earned.
The module should be available shortly in the 4.7.x-2.x tarball of userpoints on Drupal.org.
This is an example of how the new userpoints API and hook can be used in new an innovative ways.