Will there be people wearing glasses? If so then you will need to be mindfull of the reflection that can occur if the lighting isn't placed properly.
I shot a couple of family pics for my wifes family back in 2011 using two umbrellas and YN560 flashes. It was a nightmare as I was working with a low ceiling and not much clear space around me to move where I wanted myself or my lighting placed.
The umbrellas were placed in front (about 5") and just to the outside of the group in each photo. I would like to have placed the umbrellas higher than eye level to the people in the back row to cut down on the reflection of the flash in their glasses but due to the low ceiling I did the best I could. Also be mindful about shadows caused by the group in front and to the left and right of center. Again that's why I went with the setup I did for the greatest/widest dispersal of light to the group as a whole.
Others may have another way of working around a scenario such as yours like bouncing the flash off the ceiling or walls but in a pinch and having never done anything like this before it's what I came up with in a pinch.
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Here is an example where I had a black ceiling and background using the same setup as the first two shots.
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Last shot was for Mom during Thanksgiving a couple of years ago using on camera flash bounced off the ceiling.
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Don't forget to work the group as in loosen them up before taking the real photo's. When people first sit down for a pic they are usually stiff as cardboard so if there is a family joker in the bunch have them work with you on getting everyone to feel at ease while youre doing your job.
Here is one of my before shots showing how the kids loosened themselves up. Can't say Mom wasn't getting a little stressed though.

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