How big do you reckon the groups will be? And how many lights will you have at your disposal? If you can throw a light on the background you can turn it any colour you like, with a grey background being the most flexible. I've had experience with a few Lastolite collapsible grey backgrounds and they work pretty well for groups of 3-5 people depending on how you orient it and your working distance... I wish I could remember what model they are but they belong to my primary photographer and I only ever touch them when we run photobooths at the weddings we shoot, sorry
Even if you only have a key light to use, grey is pretty classy and less "fashion-y" than white. Just make sure it looks intentionally grey, not like an underexposed white background

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