If you're talking about the average book purchased from B&N, the books have DRM encryption built in. So unless you give your friends your credit card number, they can't read the books.
On the other hand, if you're talking about books from other sources without DRM (or if you've "liberated" your B&N/other purchased books)...I'm not sure there is anything to prevent it.
I don't own a NC, so I can't say I've ever tried it, though. But I've swapped SD cards between various Sony Reader's...
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