Have you built a prototype yet and determined that you've got a product-market fit?
(People seem to understand that in the world of physical gizmos, a mechanical prototype is pretty much a requirement. But in the world of software, too many folks go from the back-of-the-napkin state directly to MVP.)
Ideas are a dime a dozen. As a software developer, if you come to me with an idea then ask me to help implement it, I'm actually bringing more value to the table than your idea! Besides, there aren't a whole lot of new ideas these days -- just old ideas recombined in different ways. The value-add is in the implementation, not the idea.
Also, people are often focused on the wrong thing as their unique value proposition. I was at a meeting where a guy told us about an idea he had for a unique kind of "wired vest", and he wanted to know about patenting it. After further discussion, is occurred to some of us that while it might be hard to patent the vest, a more useful and restrictive patent might be on whatever means he used to plug the vest into something else. In other words, the INTERFACE, rather than the vest itself. Not being very technically oriented, he didn't really get the relevance of this notion.
The point is, you cannot really protect ideas. You can protect EXPRESSIONS of ideas (via copyright), DESIGNS and RENDERINGS of ideas (via patents), but along the way, the creative people you're working with are the ones expected to take your idea and get it to the point where it CAN be protected.
If you're not paying someone, I'm not sure how much an NDA is going to help, because at some point the creative person may realize there might be far more value in going another direction that you're not seeing. If they're not being paid, you really have nothing holding them back, since you probably don't even want to discuss this other thing because it's not in your focus.
I cannot tell you how many times I've had conversations with founders / managers / marketers about things I'm working on for them where I saw some possibility for the work I was doing that was in another direction and ended up getting quite an ass-whooping for having the gall to suggest something outside of their focus! These days, if I see something like that, I'll feel them out by casually mentioning it, and if they dismiss it, I'll assume they're not interested. These kinds of things are definitely NOT covered under NDA as far as I'm concerned, and they'd have a damn hard time making a claim that they are, especially after clearly and emphatically stating it's not something the company is interested in or focused on.