Instructions
Enter a battle in any of the game's modes. You can grab someone, no matter what game mode or character you are using.
Approach the enemy. You will need to be just a couple of feet away from the enemy to perform the grab move. Performing the move will leave you temporarily exposed, so it is best to attempt it when you are facing a character who guards a lot.
Press the "A" button, which is the attack button, and the "C" button, which is the guard button, at the same to reach for you opponent. If you are close enough, you will grab the enemy and he will be unable to move for a short time.
Press the "A" button to punch them while you have them grappled, or press the "C" button to throw them in the direction you are facing with the Nunchuk device.