The difference is that I'm talking about the automation creating completely new groupings, most akin to a community on Lemmy, that coordinated across multiple users, in my mind "simultaneously" with the user still agreeing to opt in to inclusion in that group.

There is an alternative way to do this, which would be that the automation groups the posts after posting, however there is a question there about opt-in, will users want to opt existing posts in after the fact?

One way that definitely would be easiest to implement would be if these groupings are essentially threads with a single piece of content as the "start" / "seed" of the thread and the other posts relating to that thread. Regarding opt-in for that I suppose it could be as easy as enabling/disabling "thread seeding"