So in ToB do you continue on with the group you had when you beat the game or are you starting from scratch? Just curious as I would hate to loose my characters, seeing as they are all at high levels.

You can start a party from scratch and play ToB only, or you can export your party from the BG2 final save into the expansion. You'll also be able to reload your final save, replay the last battle, and then go right on to ToB after the final BG2 movie sequence.