Thought I'd start a thread about army organization as this appears to be one of the more complicated (but for me one of the most enjoyable) parts of the game.
Take a look at this screenshot:

Here you see Nashville under siege by Grant, which is indicated by the orange circle to the left of his counter. BTW, when that turns red, it will be time to change Grants stance from offensive posture (orange arrows) to assault posture (red arrows) to storm the city.
Let's look at Grant's organization.
Notice in the black shaded tool tip area that he can command 25 units and presently has 19 under his command. If you look at the list you see Grant first with his rating (6-6-4) following by the strength points immediately under his command (34/34). These strength points represent the army HQ and 12lb artillery unit that was included with the HQ.
Next you see two divisions Thomas and Sherman. Notice there is no two star general in between. That is because this is Grant's personal corps. You want to use two star generals as separate corps commanders under Grants army. Returning to Thomas and Sherman, both of these are followed by their leadership ratings and strength points. Grant's personal corps of the Army of the Ohio has some pretty tough division commanders...

Now both Thomas and Sherman's division are all filled up with the 18 units alloted to a division. As I was putting Grant's army together, I didn't have any more division HQ's but I noticed he could command more... so... I added extra brigades to give him even more power. These units are listed just after the divisions in the black tool tip area.
Notice I also have Jefferson Davis in the army. He doesn't have a command, but I put him in there so that when I have a division to command, he will be ready.
Overall, you would think, oh, I've got to have everyone in divisions, but that isn't necessary true. Ideally you want everyone in divisions to get the maximum bonuses from the leaders of the divisions when you go into battle, but you don't have to have them all in divisions. What is important is that the General has the capability to command them all without incurring a negative leader penalty.
Also look at the units themselves at the bottom of the screen. Look at Sherman (10th Division) and all the bonuses he has to the left of his portrait. Now... that is what a division should look like when you finish building it... full of bonuses.
And just a couple final comments about the screenshot. You can see at a glance that Grant's army has been in a number of fights because the purple bars aren't full. You can also notice by looking at a glance that Nashville is full of troops because all the green "lights" are lite. When I put my pointer over it, it tells me that inside Nashville is The Army of the Potomac and 47 units!

Well, anyway, don't know if this is helpful to anyone... maybe others can pipe in and give some tips on organizing your forces.
...but if nothing else... it's a kewl screen

Ray (aka LAVA)