I'm glad you are finding my research useful! I'd be happy to meet and discuss the published paper and what you're trying to accomplish.
At the meeting do not reveal anything that isn't explicitly already said in the paper. Don't worry about signing an NDA at this point, or entering into a contract.
This is important, because you want to establish a relationship and rapport, and further you'd like to find out what application they are using your research in and how it applies. At this point you both need to gain more understanding and determine if this is something you can work together on, and in what way.
If they press for details, indicate that you need an NDA, and a contract, and perhaps discuss the various ways you might be able to offer your services.
Take copious notes during the meeting - who did you meet, what they said, what you said. This may become important later if there's a legal question or problem.
Then work on an arrangement you can both be happy with. If they aren't willing to enter into a contract or pay you for detailed information, you aren't out anything. If they are, then this is the best way to ensure you have a good start to the relationship.