Hi Richard,
Yes you can control a (sub)Group by a DCA.
A DCA has members which are controlled by that DCA.
A Group has channels assigned that route the audio from those channels to the group (So not members).
To see which channels are assigned to a group simply select a group and press routing next to the touchscreen.
you can also assign channels to a group here (Same for a DCA).
The other method is to press the MIX button of the group and check the channel displays for the “ON” you can toggle on off by press and hold the MIX between the faders and then press the mix of the channels you want to assign.
Hope this helps