They already stated that they'll be announcing participants in the next few months, so it may be already safe to say that the player pool is already set for the initial launch, but that doesn't rule everyone out though. Even though this starts calendar year 2018, this could apply to NBA 2K18 or 19 for that matter. Also, if it succeeds, more opportunities will open up. So whether they've already been involved or just getting started, bottom line is GET STARTED!! You never know what may come of it if players just give up and sit on the side because of their personal start point.
Now this also means that 2K Sports is gonna have to make a very prestigious quality title within the next 2 years and can't have 20+ updates that are "fixing" things yet still breaking others. Top eSports titles don't have those issues and if 2K wants to be in that number, they gotta step the product quality way up from where it is now.