In order to give a unit to another allied player, select the unit, open the Diplomacy menu, click on the 'Units' button next to the name of the ally you wish to give the unit to and confirm. The unit will immediately fall under their control and will have any orders cleared. Any veteran status the unit had prior to trading it will be lost. The only unit that cannot be given away is the ACU. It should be noted that units cannot be given to a computer player.