When you want to add a friend, you both have to add eachothers friend code. Every game as a different limit of friends you can add. After that, you can check if they are online in-game. Like a game like Super Smash Bros. Brawl, you can connect by pressing Nintendo Wifi Connection. Once you have connected, press play with friends then there will be something like FRIEND ROSTER. And that is the place where you can add your friends, check out your friend code and see if your friends are online. Yes, there is a system friend code. Go to the Wii message board and then click the address book, click the Wii to register your friend. In order to send stuff to your friend, you need to add eachother. You can do the same thing for email addresses.
Hope that helped.