For large indoor groups, a simple two-light setup is the most practical improvement over one on-camera flash. Place two flashes on stands, one on each side of the camera position, aimed across the group toward the opposite side at roughly 45° angles. That helps spread illumination more evenly from end to end.

If possible, use umbrellas to soften the light; they’ll usually give a more flattering result than bare flash for group portraits. The tradeoff is some loss of power, so keep the modifiers practical and your light placement as efficient as the space allows.

Because you need to work fast, the main benefit of this setup is consistency: once positioned, you can keep people moving without constantly changing lighting. Bare flash may give more raw output, but it will look harsher.

Another low-budget option, if the group is static and you can do more post-processing, is to shoot a clean ambient/background frame from a tripod, then make multiple exposures while moving a soft light across the group and combine the results later. That can work well, but it’s slower in post and less convenient for fast-paced event coverage.