You can only support members of your own alliance. Get the battle in your view, and right click on it. A window should come up with a button at bottom right that says "Support". That's all there is to it.
You can also left-click on the battle and have a number of options arranged on a circle, including seeing who and how many are in it already (overview) and support, which is at top right on that circle - helmet icon with a green flag.