It's "open." It's not like you can join and immediately be able to say "I'm making a mission tree for Frosthide nobody touch Frosthide it's mine I've claimed it," but the barrier to actually contributing is low enough that there are dozens on dozens of countries being worked on at once. Even if you're not trying to solo a MT, there's still a lot of room to lurk and help people tweak and balance their idea/mission spreadsheets as a total nobody before things are actually implemented.
Like the other anon said though, it's kind of a discord culture thing unfortunately. I work on a different project that also uses discord and I really do want to note that it's a fucking incredible platform to run a project like this off of, but it does make it so there's a layer of culture you have to get through on top of everything else. Is what it is.