To get other mii's, yes you have to have friends. Unfortunately you cannot play against other friends online like xbox live, you can online message, trade mii's etc...Originally Posted by hotwaterbrogden
To register other friends, you need your friend code. To find your friend code, click on the envelope on the bottom right hand side of the main screen. Then, click on the message board. A window will appear saying "this wii systems code is _____" That is your friend code. You give that code out to other people, and they will give you yours.
To actually register friends, in the message board, click register. Enter the friend code, and a nickname for that friend.