You've come to the site at a somewhat awkward time. We used to have groups that functioned exactly as you're speaking of, but due to some shenanigans all the site information was lost and it had to be rebuilt from scratch. Used to be a vBulletin forum, but that had all sorts of problems so now it's built from the ground up rather than using a premade forum software package. We're missing a lot of stuff you might expect from a forum due to this (groups, having a user profile you can customize, text color and size options, etc.) and only have the site owner, Mahz, working to add more things, but he's a busy guy so it takes time.
There are a couple ways on Roleplayer Guild currently to make interconnected things like you're speaking of. One would be to make some kind of core thread to act as the pseudo-group, then make individual roleplay threads for each area or group of people to do whatever. The other way would be to have it all smashed up in one thread but have different groups within it doing their own little missions and such; for instance, one roleplay I'm in right now has about 30 players all doing things in the same world but split up into groups of 4 or 5 that are each doing their own separate mission, and we all post in the same one thread and put the title of our mission at the top of our posts to avoid confusion.
Though this is of no help to you now, there are plans to make bigger roleplay endeavors like this easier in the future. Groups will very likely be implemented, and the site owner has talked a bit about an idea he has to allow people to make a sort of sub-site that is their own little forum (with full control over creating sections and whatnot), which would also work for what you're talking about. Unfortunately, there's not even a vague estimate of when those might be implemented, so you're stuck with some less elegant methods if you want to emulate the functions you're speaking of.
