For what it's worth, the reason I thought to automatically include everyone is that I organized these last time for my DS' DDC and no one said no. Going and asking everyone, however, was a massive pain in the butt. (So many PMs!)
The original thread did not ask people to sign up, and it was pretty clearly a positive response from the beginning, so if anyone thought, "Sure, sounds fun!" all you had to do was leave the thread alone. I wouldn't take lack of hundreds of responses as a sign of disinterest. From my perspective, there was an overwhelmingly positive response.
I personally prefer not having a thread as a sign-up list, as I favour a very inclusive approach, and there is good evidence that not everyone has time to check the forum every day, nor to open every thread, so it's very easy to miss sign-up. I will also plan to put a little text at the top to PM me if you don't want to be included, and I will take people out of the list if they don't want to participate. As soon as the first thread gets going, it will be fairly obvious, and the preamble text will explain when everyone's turn will be, so it will give people a chance to bow out ahead of time if they so desire. And again, if one (or even, in a rare event, two!) people from each group of four doesn't want to participate, it will still be a lively thread with the two or three who do.
Finally, I am totally happy (really! ) to do this some other way if someone else wants to organize it. I just proposed this approach based on my experience organizing these in the past and my personal preference to be as inclusive as possible while not making too much unnecessary work for the organizer.