It's common to have both a non-disclosure & IP ownership clause in any kind of contract you sign with contractors.
If someone is just doing you a favor (for free) it's kind of awkward to have them sign something. Contracts in general should be to the benefit of both parties.
Pieces of paper are only worth what you're willing to enforce. Are you really going to try to sue your friends for damages if they blab about what you're doing?
Trust is way more important and useful than a piece of paper - talk to them about helping you keep a secret. Show them what a (mutual) NDA looks like if they've never seen one just to be able to discuss how it's pretty common for companies to keep secrets... but I don't think you gain much having them sign anything.
Your goal is to keep a secret, not to maximize your ability to win a lawsuit. While threat of legal action can help communicate how serious you are in wanting to keep a secret, it's a remarkably unfriendly way of going about it... Find another way to keep information from spreading.
Also, and I'm sure someone is going to post this eventually, your idea isn't good enough that it matters whether you keep it a secret or not. Even though I believe this, I also think it's OK if you don't - a little unjustified paranoia in general can help avoid unexpected problems; just don't be a jerk to your friends over it.