Another idea, to implement not only Small Unit Teamwork, but also Fear of Death and Suppression:
In real life, soldiers have fear of death. There's no way that game players will voluntarily behave that way, since death costs them only a little time and frustration. Fear of death in-game would manifest as a soldier ducking whenever a bullet passed close by, or a fragments-producing event occurred within lethal distance. A soldier under sustained close lethal fire may be suppressed, i.e. he is psychologically unable to charge into that lethal fire.
In real life, soldiers have morale. It's high or low depending on multiple factors...their unit's recent battle experience (stay behind cover for a while while being shelled, and your morale goes down; your unit captures an important point of value or kills some of the enemy, and your morale goes up), whether you've personally wounded/killed an enemy lately (that's one that won't be getting you), whether someone in your unit has recently been wounded or killed, whether you and your unit are carrying plenty of ammo or have ready access to more, and so forth. Being under especially close fire definitely hurts morale...almost dying is scary every time.
So: every player gets a persistent Morale factor for when they play ground. Events and experiences in-game affect that player's Morale.
Small Unit Teamwork is implemented by Morale. It's voluntary to group up, but a soldier within X proximity of his section leader, and/or Y other members of his section, gets a Morale adder every minute if he's within that distance. Having plenty of ammo, or ready access to more, gives a Morale adder every minute.
Fear of Death and Suppression are implemented by involuntary, but morale-dependent, flinching. Immediately after a soldier with average-or-below Morale has a bullet pass close enough to actuate the bullet-sound code, or is close enough to the explosion of a fragment-producing ordnance that the code does a hit check on that soldier, that soldier automatically switches to one stance lower and loses some Morale.
Standing or Walking --> Crouching
Kneeling or Crouching --> Prone
Running --> Prone
Jogging --> Prone
Only crawling is not suppressed.
Soldiers with high enough Morale don't flinch, but still take a Morale hit. Eventually, even the highest-morale soldiers can be suppressed by continuous proximity to close lethal fire.
A soldier that's been suppressed can get back up immediately, but also can be suppressed again immediately by another event. The obvious preferred response to being suppressed by gunfire or explosions is to hightail it out of wherever you're getting shot at.
If someday Medics are implemented, having a Medic in your section is good for Morale. Having a Medic stabilize a wounded section member so he can be evac'ed reverses the Morale hit caused by the wounding.