First of all you'll need to invite them through the game's interface and exchange Friend codes. Once you've joined up, you can set up a special mock fight between the two of you, although note that you can only do this once per day per friend, with up to ten different friend duels allowed in any given day.

Fighting with friends rewards you with Friends Points which can be used to purchase useful items in the points redemption section. Can't find your Friend code? It's just off the main Menu, found under Record.