On to the most important unit in the Tempestus book, the command squad. These bring you valuable orders that really are the reason you are using this book, besides it being cool. Unfortunately they lack the senior officer special rule, a big misstep in my opinion as the army really doesn't get a whole lot in terms or orders, so using your orders and your command squad effectively go hand in hand. Unlike Imperial Guard command squads, these guys generally perform a combined role in your army.
As far as stats go, the commander has the same stats as your scions/tempestors, with a few main exceptions. He is Ld 9, has 2 Wds, and I 4, its important to remember the initiative piece when choosing close combat weapons. He has the clarion vox net special rule, which allows your squads to use his leadership for tests, minus orders. He really is nothing special really, has better initiative but otherwise has a good guardsman's profile but nothing to really write home about. He has voice of command and can issue one order. This is the important part, the one order he can issue is what is going to make your shooting phase work.
The squad itself is composed of normal scions, but can take a lot of options. Of the special options the only two that are really worth considering are vox's and medi packs. They are good, but not must takes either, vox's require an investment in most of your army, and unfortunately for the command squad they lose a weapon option to get that reliability. Which bring me to the other options, weapons. The 4 scions can each take a weapon if they did not take another upgrade, which are the same special weapons that scions can take (plasma gun, melta gun, volley gun, flamer, grenade launcher).
Put all of this together and you have an idea of what this squad should do. Use orders and clarion vox net to support your troops, so you can't stray too far. Load up on guns to take out high priority targets. They are a mix of the IG company command squad roles, not amazing at support, but still good in combat. Combined with some of the previously discussed warlord traits and you can get a lot of mileage out of them.
You should always take two, if you are only taking a single CAD. If no one is bitching too badly over multiple CADs, you should double up and take 3-4 command squads. The first one that is going to be your primary warlord needs to be flexible, so that he can move around your line and still issue orders and have effective shooting. Plasma guns are a great for this, and you should have at least one plasma command squad to deal with hard to kill targets and still be able to engage at range.
Your other command squads should reflect a similar role, but can bring different weapons. Ideally you want to use weapons that you are not going to really be using orders on, so that your command squads can issue them to your line squads. I like the volley gun for this reason, its a good anti infantry weapon and doesn't require much support. Melta guns are good as well though you will be tempted to move alone with them to get into range, don't unless you have to, let them come to you.
Overall using a command squad is a balance of firepower and orders. Keep squads close to them to both defend them and benefit from their abilities. The command squad should not be operating on it own, use it in conjunction with your army. Orders should be used on your main squads, though it is ok to use them on your command squads when the benefits outweigh it such as with a plasma squad.
These guys are the cornerstone of your army and are what make your standard hot shot be able to kill just about anything in the game minus AV13 and 14. With them, there is nothing you cannot kill.