With DDP, an 18% service charge will be automatically added to your check if a) you have six or more people in your party (regardless of how they are paying for their meal) and b) any time you order non-DDP items like adult beverages, appetizers, etc. These are not optional.
If you have fewer than six in your party, and don't order anything outside of DDP, nothing will be added to your check but you are still responsible for whatever tip you decide to leave. The gratuity is no longer included in the price of DDP after 1/1/2008.