Digital cameras make it relatively easy to shoot available light in almost any location. But a few simple techniques can make a good situation better.
If people are sitting at a table with poor light, try moving them to a table with better light. After a few moments they'll pick up on the conversation where they left off and you now have them in light that will work for the photos. If shooting for advertising or corporate it may be perfectly fine to alter reality. However, if photojournalistic coverage is needed, manipulating the environment may be inappropriate.
If the available light is not right, try using a reflector to help improve the mood. It’s much less intrusive than flash and would supplement the ambient light. Have an assistant hold a reflector just out of the view of the camera. Bounce the light back into the subjects face. It would help to draw attention to the main subject.
One could use either a constant light source or flash to light the subject. Use spotlight effect as much as possible, rather than floodlights where everything is lit equally.
Another technique of the trade is to place a colored gel over the background light. It will simplify junky background by blending the background to one color. At least two lights are needed, one for the subject and one for the background. The light on the subject should be brighter than the background light.
It’s not necessary to use a lot of artificial light…just enough to supplement what’s already there.