# Boolean modifier on groups - can't select 2nd object - is this possible?

I'm making a unit cell of a face-centered-cubic crystal, and I want to cut a group of icospheres with a cube. I have grouped the spheres. Then I selected them by selecting one, and with the select menu, choose group -> siblings.

Once I do that, two things happen. 1st, other objects are selected at the same time (have to de-select them with shift-click) and now the cube no longer appears in the choice of objects as the second object for the modifier, only the icospheres in the selected group. See screenshot bottom right (below). (Ignore the un-applied wrench-icons in that screenshot)

UPDATE: Please let me know if what I'm trying to do is not possible!

Maybe I'm doing something silly, but I can't spot it. If there is a step-by-step out there that applies to groups, I haven't found it yet. Using 2.74.

Of course, later I'll do it with script, but I really should know how to do it by hand if I'm going to respect myself in the morning.

This is what I'm after:

import bpy
import numpy as np

icorners  = [(0,0,0), (0,0,1), (0,1,0), (0,1,1), (1,0,0), (1,0,1), (1,1,0), (1,1,1)]
corners = np.array(icorners, dtype='float')

isides = [ (0.5, 0.5, 0.0), (0.5, 0.5, 1.0),  (0.5, 0.0, 0.5), (0.5, 1.0, 0.5),  (0.0, 0.5, 0.5), (1.0, 0.5, 0.5) ]
sides = np.array(isides, dtype='float')

scale, size = 3.0, 0.5
offset = np.array([-0.5*scale, -0.5*scale, +1.0*size])

for corner in corners:
location = scale*corner + offset

for side in sides:
location = scale*side + offset