If I understand your question correctly you don't want your group to remain standing. You rather want to prevent them from lying down, right?
That is not possible, to the best of my knowledge. The only workaround I know is to set their behaviour to AWARE (it makes them less prone to go prone than COMBAT mode).
There is a problem witht the setUnitPos-command. It locks the unit in the standard animation connected with the behaviour he was in when he first recieved the setUnitPos command. i.e if you give it to a loon in safe mode, he will usually remain standing with his gun on his back even if he spots an enemy. I discovered this quite recently, and it needs more testing, but I'm pretty sure it works this way.
I usually start my loons off with a setUnitPos "UP" command in AWARE mode, and gives them a setUnitPos "AUTO" when they get in close proximity of the enemy, using a repeating trigger.
Example: (the group is already in setUnitPos "UP"-mode)
Condition: leader sarge1 distance enemyGroup < 150
On Activation: {_x setunitpos "AUTO"} foreach units group sarge1
On Deactivation: {_x setunitpos "UP"} foreach units group sarge1
The exact details, distance, etc depends on the mission at hand. But if you play around with something similar I think you might achieve the right effect.