I think this is a real shortcoming in the Ashes interface; so much so that I wonder if I'm missing something. But it seems simple:
If a group/army is already selected, I can add a unit to it by shift-clicking on that unit. BUT I cannot remove a unit from a selected group by shift-click.
This is not only counter-intuitive but really inconvenient. As things are now, to remove a unit, I have to select the army, disband it, select that unit, move it out of the way, then re-select what's left of the group (being careful not to pick up that unit or others) and re-form the army. This is just crazy in the heat of a battle.