I suggest sub-prizes be awarded when you hit certain fractions of the necessary friend count. For example, the first time that your friend count is >= 1/4 the leader’s friend count, you award 3 BPs randomly. You would do the same at 1/2 and 3/4. If you re-hit a previously hit milestone (because you lose friends or the leader gets more), you don’t give out more BPs. This would encourage people to join by providing a chance at quicker gratification.