Hi,
I would approach them as if you are just about to launch your service/product, mention that his/her 2 main competitors have already signed up (assuming he/she doesn't have any contact with them) and ask him if he wants to sign up for the free trial period (no strings attached). Then, tell him/her that seeing how you are still making changes to the product/software, ask them what features/functions would be most important to them.
This way, you not only get feedback from your target customers, you actually get potential pre-signup clients.
To be fair to that person: if they do sign on, you better over deliver on your promises. If you decide not to move forward, contact them, thank them for their time, and tell them that you will not be moving forward with the business/product/service.
I've helped hundreds of startups with this question/stage, and I'm more than happy to point you in the right direction.
Best of luck