First of all: At Leanpub we think the best thing to do is to just publish. Your actual readers are the best reviewers. If you want other reviewers, you can just give them a free coupon.
Now, if you want to get feedback before your first publish, you can do this:
Generate a preview at https://leanpub.com/YOURBOOK/preview
Once that's done, that page will show links to the book in PDF, EPUB and MOBI
Share those links with your reviewers
In terms of copyright, by default your book has a standard copyright license when it is published. You can also select a Creative Commons license if you choose.
In terms of NDAs for reviewers, I'm not a big fan, but you can do whatever you and your reviewers find agreeable :) Getting someone to review a book
before publication is a huge favour, so my own position is that getting someone to sign something onerous is an imposition on an imposition.
If you're concerned about your ideas being stolen, the best thing to do, in our opinion, is to publish.
My longer version of that argument is here:
https://leanpub.com/lean/read
https://www.youtube.com/watch?v=ozO0kOnqmyA