To my knowledge, at the moment, the only way to give a an order to a subset of units (let's say a brigade) is to "sacrify" one of your eleven "chef de corps". Only 11 are authorized at the moment. May be a simple way to have brigades would be to have more CC. But still their AI will be the one of the corps. Let's wait a bit. I am sure that JMM will make us happy but no pressure.