Inside the booth
Photobooth shadows from overhead lighting
Move the setup before trying to repair every face.
A ceiling lamp can leave eyes in shadow even in a room that feels bright. Read the test portrait instead of judging illumination from the room alone.
Look at the eyes
Take a quiet sample with the person seated where guests will sit. Check eye sockets, the area under the nose and shadows behind the head. Compare a second sample after changing the camera-and-seat position. Keep the expression similar so the lighting difference remains easy to judge.
Change one source at a time
Try a more even front-facing light or a position nearer suitable daylight. Avoid placing improvised coverings on hot lamps or blocking ventilation. Use lighting equipment as instructed by its manufacturer. The aim is comfortable readable faces, not the largest possible number of lights around the booth.
Retest the whole frame
Check glasses, backdrop and people of different heights. Improving one face can create a bright reflection elsewhere. Save the approved lighting arrangement in a setup note, and test it after any furniture move. Do not rely on correcting every guest’s strip after the event.