Jimmer is obviously the main attraction, but Harris and Honeycutt are legit 1st round prospects as well, although not in the lottery.  Harrellson, Muldrow and Culpepper will all probably go undrafted and are likely here to act as bodies in 3-on-3 matchups.  

My best guess is the Kings want to see how Jimmer handles being in a facilitator role and also how well he plays in defense, thus the group workout.  He can't really answer any questions by working alone.  Culpepper is an aggressive scoring guard, and he'll test Fredette on that end.  

I wouldn't be surprised if Kemba Walker also is in some sort of group workout when he eventually comes to Sacramento in order to answer the same questions.