We love kids, we really do, but we would like the grown up's at our wedding feel free to relax and cut loose without little humans running around.
If your invite said "and guest" then yes, absolutely, please RSVP with your added person. If your invite did not include a guest, please refrain from bringing extra people and adding an extra cost.
Semi-Formal: Tux's and gowns are great, but suits and cocktail dresses work too!