I mostly write for the catalog now, but I have one client that sent me a lot of private requests throughout last year. I only do a few articles for him each month now, because the main project I was working on with him is finished.
I've never been invited to join the writing pools, but it seems like that's how some writers get more consistent work on the platform. I've sold roughly 50% of all my articles in the catalog, and I try to add 20-30 new articles each month.
It's also possible that Constant Content will reach out to you to work with a specific client if you've submitted a few articles for a specific category and they have a need for a certain project.
Anyway, I hope this information helps!