NS's offer to build your site is just a gimmick. If you want a couple of html pages of about us, contact us, homepage, etc. it would be fine. But I can't see any startup that can use that type of service to build anything meaningful. I would definitely stop talking to NS immediately.

As for your programmers, NDA would have helped. But also don't lose sleep over it. I am a programmer and have worked with many other programmers. So far, I have not seen one person stealing an idea from a client due to the lack of a NDA. Maybe I just hang with those with moral integrity.