Create ten players. Assign all ten players to a Created Team. This duplicates the ten created players. Now delete the ten created players. You have ten more Create Player slots, and can just assign the players on the Created Team to whichever team you want them on.
In regular 24/7 mode, beat any of the following modes to have it unlocked for multiplayer games. On your 24/7 screen; pick a battle ground. Once you beat the opponent; you will now be able to use it play against a friend.
In 24/7 mode, defeat the final International boss. Your 24/7 character will no longer suffer deterioration in his statistical attributes, no matter how long you go between playing the mode.