When you’ve found someone who seems like a decent candidate so far, make first contact. (The contact form on the website is fine as a first step.) Introduce yourself and state what you are looking for (a developmental critique, copyediting, etc.). Give a very brief description of your project, including its genre and approximate length. Give an idea of your planned next step and any deadlines (even if only approximate ones). For example, have you already signed a contract with a subsidy press for publication next fall? Are you planning to submit the manuscript to publishers and/or literary agents after editing?
The job board is NOT a big reason to join the Den, especially if you’re a new writer. It’s a higher-end board that doesn’t post any low-priced gigs, and is a good resource for mid-career writers. But in general, the board is a real sidelight to the point of joining the Den, which is to access 300 hours of training+, 24/7 forums for support, live events where you can ask questions of experts, and more.

Obviously there is a big difference. I actually started with screenplays (with some mild success) partly due to hearing that as long as you have a good story, the grammatical errors won’t be a big deal. Well, that was not true. Maybe if a producer liked the story or I was already established but every contest I entered always came back with feedback about the grammatical errors. Even if I won or placed as a finalist, I always received critique on that. Very little is spelling error, it was more about structure, etc.