Yes, you only need one Non-Disclosure Agreement signed by all 9 of you. My biggest concern, though, is whether or not the NDA is adequate to protect the sensitive information.
Frank W. Chen has been licensed to practice law in California since 1988. The information presented here is general in nature and is not intended, nor should be construed, as legal advice for a particular case. This Avvo.com posting does not create any attorney-client relationship with the author. For specific advice about your particular situation, please consult with your own attorney.
The investor is the important party who executes the NDA. An authorized representative of the company can execute the document, acknowledging its receipt, and the company's reliance upon the NDA. Most NDAs are poorly drafted. Be careful.
Each of the eight of you should also be executing a PIIA (proprietary inventions and information agreement) with the company. Your failure to take this important step toward prudent management of the company's intellectual property could have dire consequences.
Hire legal counsel immediately. You can pay a relatively small amount now, and avoid paying a huge amount later.
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I agree with my colleagues. You only need one, but a good one. Boilerplate NDAs are particularly horrible in terms of "off the shelf" documentation because they just cannot predict the consequences of facts that an interview with an attorney would uncover, and thus deal with. The legal versus the psychological benefits are a different issue.
The above is general legal and business analysis. It is not "legal advice" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
If you haven't yet formed a company and issued membership interests to each of the 8 people, then yes, all 8 people can sign the same document as the disclosing party. However, note that many potential investors will push back or negotiate the NDA, as the document may make it difficult to seek out investment opportunities in the future, particularly if the subject opportunity is defined in very broad terms.
With the above in mind, wise to coordinate with an attorney on drafting and signing an NDA so that you don't make a costly misstep.