Multiplayer variants of Bethesda RPGs are modders' Everest. Almost insurmountable, but oh so in demand. Imagine it—wandering the Wasteland with companions who don't get stuck in doorways. The stuff of fantasy. Morrowind multiplayer is making headway, and now a mod for Fallout: New Vegas is seeking beta testers to quest together.
Developer nvmp_corporal explained how NV:MP works on Reddit. Quests are still handled client-side; that is, each player retains their own quest log. However, NPCs are synchronised to a host player in any given region—if the host caps a quest giver, he'll be dead for everyone else in the area, forcing relevant quests to fail. This could break friendships. Once they leave the area, a new player becomes host and NPCs re-sync. It's messy, but clever nonetheless.
VATS is disabled outright (slowing time for one player would cause the server to wet itself), but the inventory works almost as normal, with the exception that items are stored server-side and you become invulnerable while it's open. As this is beta, you should expect saved states to be wiped now and again.
More interesting still, mods that don't add new regions or items to the game ought to be compatible with NV:MP, although the New Vegas DLC won't function as yet.
The next test is in two weeks time, and you can sign up at this flashy site.