We do recommend peeling most of the fruit before adding to your blast. Simply dropping a whole, unpeeled lemon into your NutriBullet would make for an uneven texture at best, and possibly damage your machine. Here is the peeling technique we recommend:

Cut off the two pointed ends of your lemon with a sharp paring knife so they are flat. Stand the lemon upright on one end. Use the paring knife to cut the peel off from top to bottom along the curve of the lemon in small strips, taking care to cut as little of the flesh as possible. Continue around the entire lemon until the rind and most of the white pith is removed.