Incorporate Ambient Light
As you shoot your couple in an outdoor setting at night, you will need to rely (in some way) on the existing light in the scene. Take a look around to see what type of lighting is available to you without including any off-camera flashes. You may find that it is not completely dark when your shoot starts, so there is a bit of light coming from the sky. If the sky is dark but clear, take advantage of any light from the stars to create wedding photography. Since the shoot is at night, it is more likely that your couple chose a location that is close to buildings that may supply a light source or even a location that is close to street lights. Shooting with ambient light, especially at night, can give the image a sense of realism.
Identify Surrounding Light
If the reception is outdoors, it is likely that there will be lights to help the guests see during the festivities. This is something you can use to your advantage since the lights are unique and will set the tone for your images. It may also be helpful to you because the surrounding lights can take the place of any off-camera lights that you brought for the shoot. If there is a dance floor with lighting surrounding it, place your couple in the center and shoot wide. This allows you to focus your camera on the bride and groom, include an intriguing background, and eliminates any harsh light that comes from the site.