NDA for private alpha/beta users?
I'm about to open my app to a group of selected persons to take part in private alpha/beta testing, Should I require them to sign a NDA to particpate?
You want virality and promotions, not secrecy.
The only way I could see this being justified, is if the product is for government (in which secrecy can be important), or big enterprise (in which you're going to have a few clients to start, and you're competing with some massive companies and don't want to tip them off any sooner than absolutely necessary; signing an NDA still won't go over well, many will refuse). If this is in the consumer Web / app space, or small business space, absolutely never seek an NDA.
If you want to keep it secret, and the product is very alpha, test it with the smallest circle possible, ask people nicely to keep it under wraps; you can even lock the access down tightly, but have no illusions about people sharing what they see or talking about it. As the product gets closer to launch, open it to an ever wider audience.
Second, it'll be almost impossible to enforce. NDAs are a big pain to enforce under the best of circumstances.