Here's a simple two step process to becoming a wear tester for running shoes.
Step 1, you go to this site - http://weartest.newbalance.com/ - and sign up. This involves inputting some detailed information about your height, weight, foot widths, sizes, your running routines, etc.
Step 2, you wait. Patiently. Just because you signed up doesn't mean that they are going to start sending you shoes right away. It might be a few weeks, or even a few months. Their initial testing is primarily targeted at men's 9.5D and women's 7B and you have a better chance if you happen to be one of those.
The cool thing is you get to wear and test out shoes before they are available publicly. You get to keep them for a few days and run in them. At the end of the test, you return them, along with a detailed questionnaire about the shoes and your experience with them.
This is specifically for New Balance since that was my experience. I got to wear test the MR805. This is a lightweight trainer and meant primarily for interval training and speed work on the track. I liked the grip very much. Tested it on both the track and on wet roads and the grip was excellent. It's lightweight shoe and I'd definitely recommend it, based on my limited experience.