Once you have completed a game (whether you win or lose) you will automatically be placed in a group. Each group contains 1000 players who will be competing against each other throughout the duration of the World Cup. 

Note: If you are in the middle of a game when the World Cup ends, this game will NOT be counted towards the final results (the timer counting down to the end of the Cup can be found on the lower right corner of the World Cup leaderboard).