Business dinner - 10-12 people downtown?
If you are inquiring about venues near the Miracle Mile you would need to ask on the Los Angeles board, but assuming you meant the Magnificent Mile here are some nice venues in the downtown Chicago area with smallish private dining rooms:
*Embeya (Modern Asian Fusion)
*David Burke's Primehouse (Steakhouse)
*Sepia (American/Casual Fine Dining)
*Sable (Craft Cocktails & Upscale Pub Food)
*Sixteen (Fine Dining)
*Tru (Fine Dining)