Instructions
Complete the first available level, "The Magic Begins," then start it again in "Free Play" mode. Select either any adult character or wait until after you have completed the story event in the fourth year involving dragons to select a student. In the fourth year the students learn the "Reducto" spell, which adult characters already know.
Play through the level until you reach the underground vaults of Gringott's Bank. Go all the way to the right, past all of the vaults, and you'll see a locked chest on the right.
Hold the "C" button to bring up your spell wheel and select "Reducto" from the options, then press "B" to cast the spell on the silver lock on the chest. Collect the Griphook character token from the chest and finish the level.
Go to Diagon Alley and enter Madam Malkin's shop. Go up to the counter and it will show you all of the characters that you can purchase. Purchase Griphook for 55,000 Lego studs to unlock him.