Not sure if this is a bug or just not implemented yet, but I have the Home Harem with Sasha, Bree and Minami in it. (I'm at the stage where I need to ask the other 3 if Sam can join, and have had the 3some with Sam, Sasha, and Lexi)
While Home, if I ask Sasha or Bree for Sex, the one I don't ask along with Minami pop up asking if they can join, however I only get the option to include Bree and Sasha, (or just the one I initially asked), and Minami is left getting a -1 affection hit regardless of which option I select.
If I ask Minami, the other two ask if they can join, but I only have the option to sleep with Minami, and the other two end up with a -1 affection hit.