We don't have any kind of formal sharing, lending or donating mechanism.

In terms of passing a book to one colleague at a time (the way a physical book can be shared), the cumbersome nature of sharing physical books vs. the trivial nature of sharing ebooks means that physical book metaphors don't necessarily apply: reduced to absurdity, by this logic one person at a large company could buy a single copy for the entire company, and we obviously don't support that.  Now, some companies try to "fix" this with DRM, but we don't believe in DRM, and have no DRM on our ebooks.
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Now, when libraries, professors or organizations contact us about buying multiple shared copies for students or employees, what we do is facilitate a discussion between them and the author(s), where they say how many people will use the book at once, and the author(s) name a price.
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So, the short answer is: Leanpub books are for individual use only.

Now, if you're done with a book and are going to "hand it off" to someone else regardless of what we think, this is something which is done independently from Leanpub: please don't tell us, and especially please definitely don't give it to more than one person.  While some authors may be fine with a book being passed along to one other person, many would not be fine with their books being given to a handful of people.

Finally, since we're a self-publishing platform, you can always ask the author what they think. Many authors have an email contact form on their book pages, and/or have Twitter accounts where they can be contacted...