My understanding of how it is working in CW2 is word-for-word what Orso said.
. An Army GHQ force will never apply an Assault or Offensive posture in a region if there is another friendly stack in this region.
This really is just an implication of the more general statement of the rule in Orso's first bullet point, but it is a very important practical takeaway. Let's say you are besieging DC, and you have two Corps and an Army. The Army will not Assault the structure with the other two Corps no matter what orders you give it, since they cannot target another stack. Similarly, shuffling heavily damaged divisions into the Army stack in this situation would mean they are prevented from fighting and recover cohesion and hits unmolested.
Another implication/consequence is that under the normal rules Orso refers to in the second bullet point, like any other stack Armies in Defensive posture CAN enter on the second and later rounds in support of "overmatched" friendly Defensive stacks. This is based on a different criteria than MTSG, (150% of PWR or something) and it is common for battles to occur in which an Army might have tipped the scales but didn't meet the threshold to enter the battle.
So, ERISS, I would agree that they are BEST used as MTSGers, but they CAN, under certain conditions, participate in defensive battles with other stacks present.
Since I am a CSA player and often do not have enough Corps Commanders, I regularly cram Army stacks to their CP limits and use them as effective front-line combat stacks. In order to get around the Army Participation rules, I avoid situations where they might enter combat when there are other stacks present, and when I can't avoid it, I either consolidate to just the Army stack, or maximize Corps stacks and minimize the Army, so that as many troops as possible can participate. I use them exactly the same as any other Corps in situations where I want to spread across multiple regions and have them MTSG (although I make sure they are the only stack in their specific region).
Armies are likely to MTSG, and if they enter a large battle with just a brigade or an under strength division they will be in big trouble since they will not be big enough to fight a round on their own. Having too small of an MTSGing stack has cost me in battle after battle, and is something I carefully avoid (a full division is sometimes enough). Consequently, if I am not outfitting an Army as a fighting stack, I don't put anything in it at all: having just a few combat elements draws them into battles they are too small for, artillery would be better off in a stack that can fire during the first round, and any other support elements in the stack aren't benefitting fighting troops. That means that Lee is either decked out with six divisions an HQ and a Balloon, or he is all by his lonesome (or babysitting a bunch of unused 3-1-1s). I have never ever lost an Army commander in actual combat, (not saying it couldn't happen) and leader-only stacks are almost impossible to force to battle anyway, so I think it is perfectly safe to use a "naked" Army set-up.