I think the Guide as it stands is good, it is not very hard to read and understand and I’m pretty sure if you want to be a tester you’ve got to be willing to read through information about it!
I think having that link in lots of places is the important thing and @Ian_Chimp 's thought of a direct link from the shopping site is a very good one.
Maybe just something at the start saying, “All information you need to join the testing program is here, please only email me directly with your sizing information. Any other questions feel free to ask them in Sex Toy Tester - Chit-Chat where established testers can help guide you.”