I think a lot of the posts, especially recent ones, are goofing around waiting for plot to happen. There is a brief(ish) explanation of lore/plot in the OG post, but all I really know for the last hundred pages is there’s a couple evil multiverse empires we want to stop, and currently we are on Elantris and were trying to stop Wyrn, a version with the support of these multiverse empires. Right now a bunch of new people showed up (10 of which are myself... don’t ask) and we’re just having a load of character interaction and good times while we wait for the thread to split into a few, to refocus on plot. I don’t know if that answers your question.
To join, simply just have a character show up. Random multiverses portals are common for this very reason, and half the characters have just fallen out of the sky randomly (I like to say that the reason is because there isn’t high multiverses activity, but there’s no real worldbuilding reason I know of).
I think the Macrocosmos is fairly infinite, and this technically any world is possible. A world could be as you say simply because it is so far into the infinities that no one has ever needed to touch it. I think, however, it seems that it is technically possible for multiverse-hoppers to hop to any universe within the Macrosmos, if that makes sense. I don’t think or know of any way to shield from that.