Question

I can follow Section 5.3 to build the subgroup but cannot assign it to a specific fader for control.

Answer 1

If you created the sub-group using the "Mix" buttons - assigning a Mix button as a Subgroup (subgroups are Blue), the "Mix" master fader is the fader to use for the sub-group (the main fader for the "Mix" you selected).  To access this fader, all you need to do is select the "Mix/FX Master" section (on the right of the console), and you see the "master" areas where all the master-faders reside.  Your sub-group "Mix" fader will show up in that area.