The group has an imaginary leader in the middle of the group for high level pathfinding (areas), for low level pf (tile) the first line of units are leaders, others are moving relatively to them, but taking into account tile accessibility too. Thus I can later set group orientation and different formations by assigning leaders and desired leader positions. This is the plan laugh atm they have constant orientation.
I'm just imrpoving group collisions: in some cases (when 2 moving groups are crossing each other) 2 units could enter into the same tile, so a 3 stage avoidance is appliad at the moment (1-turn away and slide on each other, 2-new path towards another tile, 3-set unit to passable temporarily if nothing helps). But later it should be improved with a unit priority based pushing system... (e.g. because of siege units, vehicles, or cavalry)
But in reality crossing of group of soldiers, or changing formation also resulted in broken formations, thus sometimes resulted in loosing the battle, so I think it should not be perfect and easy task in a game too. Of course, it can be annoying if the solution is too stupid.
Targeting is group based, updated in cycles, so following a moving target hopefully will not be a serious problem. I also hate when it is stupid, so that will require some enemy path estimation (e.g. by stealing a few steps of its path).

Free world editor for 3D Gamestudio: MapBuilder Editor