My plan will be as follows:
Build a simple system to offer an invite code.
I add them to the system, and see if they take up the offer.
"The invite code" will be going in emails specifically written to them.
Who is them?
Family and friends, but not everyone. People who have a specific link to the web app in one form or other.
ie, They work in that field, they have an opinion on that field, They have knowledge of that field.
Once there, my next attack will be to "research" ( more than just google ), low and high level bloggers on the internet who might fit the initial requirements. ( In the area, opinion etc )
Build up a short term relationship ( if need be ) and write them a personal email. ( Not generic ) Further to that, if any of the people I research have a post address, I do plan on paying and sending them a letter. ( written by my own hand, not one of these email to mail apps )
When I finally start this process. I will do my best to keep records of it and then maybe one day convert it into a blog post.