There are alot of folks suggesting zooms that do not get you to the preferred portrait rage of 80-120mm. 70mm is not going to give you the look of a 105 or even 85. Your going to have to stand too close to your subjects. This will make their faces look fat. You dont want that.
Id suggest the 24-120 if its your only lens. Maybe the new 70-200 F4 if your looking for a lens specific to shooting portraits.