Very large groups of people, and crowds, are rarely under the control of a photographer. Lighting is almost irrelevant because nothing much can be changed. It is just a matter of finding your spot and doing what you can. An elevated position helps to show depth and more of the people in the crowd, but may not be achievable. However, the largest events usually have a stage or focal point that provides opportunities. At pop concerts, for example, the most animated and enthusiastic fans are at the front so get among them if you can. Watch your camera, because quite a bit of water flies around.
If a group is moving, as in the case of a public demonstration or parade, try walking backwards with the flow of the crowd. Use a lookout to watch for hazards, and remain sensitive to the purpose of the event. Alternatively ride on a vehicle, or find a convenient corner or doorway and let the world flow past.
When photographing a section of a large crowd it is difficult to achieve a clean cut-off at the edge of the frame. Disembodied arms and legs creep in everywhere, and may look untidy. An integrated group can sometimes be isolated, but don't rely on it. This raises the issue of whether people should be cut in half at the frame's edge. The purists say this is undesirable, but it does result in some visual tension - perhaps a feeling of action or pent-up emotion. If so, it may make the image.