From what I can tell, adding friends involves recruiting them via a link, or going to an existing user's hero page and clicking "Yeah, this is my friend."
The former makes a good bit of sense--somebody who's registering through your recommendation is bound to be a friend, but that second one seems rather... Easy. Does that actually add the person to your friends list, allowing you to request batteries and...
...Actually, what else are friends for, other than keeping track of cool comrades and requesting batteries?