Many...probably most...progressives advertise their openings in that forum. But not all. I run a league (the NWP-RE) in which most of our very few openings across 28 seasons have been filled by word-of-mouth recommendations from current owners. There are a few reasons for this, but paramount is that the structure of the league is significantly more complicated than most progressives, and I only want experienced high-quality owners to join.
I'll offer a couple of recommendations to you as a new progressive player. What follows is just my opinion, so please take it as such.
-- Get things in on time. Keeper lists, draft picks, entering your team in a new season, etc. Nothing frustrates me more as a progressive commissioner than having to chase people down to get stuff done. Being a commissioner takes a lot of time, especially if you care about doing a good job at it, and I consider it very unfair to me and to the other owners if someone consistently is unable to meet deadlines. We're all busy - jobs, families, lives outside of WIS. But if you can't commit to meeting deadlines, then progressives probably aren't for you.
-- It's your responsibility to read and know the rules. Most progressives have a league constitution or similar document. Make sure you read it thoroughly and understand it. And do not assume that the way one league does things will be the same in other leagues. Things like rules about trading, how the draft order is determined, what constitutes "tanking" and what the consequences are...all of those vary from league to league.
-- Speaking of tanking...don't do it. Most of us define tanking as "deliberately managing your team in ways that make you less likely to win." And it's unfortunately not uncommon to see it in progressives, especially ones that use some sort of reverse-order-of-finish draft. There are gray areas, but experienced progressive owners know all the tricks, and know how to spot them when they are tried. Note that I do not consider "having a bad team" to be tanking. It's very standard practice when you're rebuilding to trade away guys with current value for draft picks or guys with future value, much as in real MLB. Having a team with 50-win talent is not tanking. Managing a team with 50-win talent to a 30-win season because you didn't put any effort in, or deliberately sabotaged them...that IS tanking.
-- When you join a progressive, you are making a commitment to the commissioner and the other owners. Don't drop out just because you have a lousy team, or a rules decision didn't go your way. And it's generally good form to give the commissioner advance notice of your intention to leave. Dropping out right before the draft is about to start is really uncool.