DFTerm.
http://dwarffortresswiki.org/index.php/Utility:Dfterm3It allows multiple users to grab control of a single remote terminal, through which they can manage an instance of dwarf fortress.
It has some quirks, but it works.
I tried to host a DFterm server once, but my upload pipe was too weaksauce to host properly. Way to go american ISPs. Somebody hosting a DFTerm server needs to have a computer beefy enough to run DF at appealing speeds, *AND* an upload pipe capable of sending rapid telnet traffic to multiple connected users. I have the computing hardware available, (Even have a genuine datacenter server I could use now!) but I lack the bandwidth.
Still, this is a way you can do a genuine multiplayer DF fortress.
As for "Multiple simultaneous* overseers working at the same time*", that is something that DF is not designed for. DF is not a MUD, and does not have any code to run as one. It would be interesting to see toady make such a thing, but as I understand, he is not a unix/linux officianado, and has assistance with his linux and OSX ports.
(I really think a fully network daemonized version of DF, a-la how NetHack originally worked, and how Rouge originally worked would be a thing of unimaginable awesome-- because it really could allow some serious crazy to happen-- such as multiple user fortresses in the same world working simultaneously at different sites, multiple overseers in a single fortress, and random adventure mode players wandering through actively administrated fortresses-- but with DF's current process model, it would require a super computer from an alien spaceship to pull off. A fully MUD style DF would be crazy-cool. Sadly, DFTerm does not provide this. More, what people did with friends in the 80s and 90s with single player video games, trading the controller around.)