The easiest would be to set up calibre on a laptop, and run calibre server on that over wifi. Those with Android or iOS should be able to connect and download over wifi. Mail the books to those with Kindle. The others would have to bring USB sticks and use sneakernet to share.
But this would not be possible for most recent books due to DRM and copyright restrictions. But there are plenty of books that are free to share available here on MobileRead for instance.
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