To send reinforcements to someone, you must click on your rally point. Look for the "March Troops" tab, click there. You will see a drop down block at the the top, in the middle. Click here and you'll see options such as attack/transport/reinforce/reasign, etc. Click on reinforce. Now, under that you'll see all the available troops to send and the amount you have of each. Choose what you want to send and how many. Then, fill in the coordinates where they are to go and then click on the button at the bottom to send them on their way.