Hey guys. I have just been playing the demo, encountered all the same problems you have, yet i have figured them all out and the game is brilliant.
You must buy units as the AI commander builds base structures. Barracks gives you general soldiers, Helipad gives helicopters etc. You must gain money to BUY units by gaining kills. push enter or scroll button while in your hq to buy units and weapons depending on the structures present.
you can control any unit you purchase, i dont know why someone above mentioned they werent able too. confusing.
The reason the units you buy get massacred so easily, is because you have not bought a fire team leader or a squad leader to move with them. If you do this you can hear the fire team leader or squad leader barking commands and keeping them on their toes as they move and they are extremley efffective. It enables them to move properly, attain the correct stances and pull back when in trouble and defend each other without you having to do it with complicated commands.
How to change to commander : enter map, click on "voting" and click your name. As commander you are in charge of base building, so you can buy the structures to gain the units you want without waiting for the AI to decide its the right time.
Destroying the base is BIZARRE. for some reason my helicopter says "negative" when i order him to attack hq, as do my AT specialists, yet i can destroy them with one SMAW shot. It is a pain as you have to keep returning to rearm every pot! Im sure this is not the case in the full version, it must be a pre release bug.
One thing that gets me is the abilty to get your team to rearm from the supply trucks. They shout "OUT OF AMMO!" but refuse to grab more from teh truck parked next to them. any help on that would be great!
Hope this helped. Tom