Also notice that the area, A, of the bolt will cancel out of the shear stress due to torsion equation and not affect the shear force (which makes sense). That looks to be the way the NCEES solution did it. Since the bolts all have the same area and same distance from centroid, J = Nr^2A (Where N is the number of bolts).
When calculating J and using the parallel axis theorem, the very small Jc value of pi*D^4/32 is ~ zero. So you are only left with r^2*A as the meaningful value. And this means that N should be 2 in this case, because the center bolt J is ~ 0 (As its axis lies on the bolt group centroidal axis).
Hope I didn't confuse the hell out you. Check out the MERM equation 53.64, but especially read the paragraph above the equation.