Using that lens at f4.5 on a slow shutter speed and only using a single flash for such a large group is asking for trouble.
If you shoot lots of indoor groups like this I would invest in a couple of Bowens studio flash heads or similar.
That way you will be able to use a much lower ISO setting and stop the lens down a bit more, f8 or f11 should be ok.
At 24mm right at the end of the zoom your bound to be getting falloff at the edges.