Use the stage wash lighting as your main light and keep the setup simple. Ask for a single warm white wash and have all the wash lights turned up to maximum or near maximum. Before the group arrives, stand on stage and meter/test several spots to make sure the light is reasonably even across the whole area. Set a custom white balance from the stage lighting if possible.

Shoot from an elevated position, such as a ladder, so people aren’t looking down at the camera. A slightly higher camera angle is generally more flattering for a large group.

For lens choice, use the longest focal length that still fits the entire group/stage in frame. Avoid going too wide, especially if people will be near the edges, because wide-angle distortion can make them look unflattering.

Use a tripod. That lets you choose shutter speed based on subject movement rather than camera shake. Around 1/30s may work if the group stays still, but take multiple frames at each setting. Use manual exposure rather than auto, and check your histogram. Test beforehand at the location if you can.