DESCRIPTION
You can enable Display Mirror Plane on the bottom to show the local mirror plane of a group (the mirror plane at the root level is always at the object origin).
When a set of strokes are grouped, the group pivot will be automatically set to the center of the selected strokes. If the selected strokes include mirrored strokes, the pivot center will be set at the global mirror plane.
If you want to manually set the group pivot, you can select a stroke within the group, and then press G on the top bar (T is for Target pivot and O is for object pivot).
Then you will see that the local mirror axes overlap with the selected stroke axes.
May develop some gizmo tools to explicitly change the pivot of a group in the future.