Not sure if that'd be exactly possible. You could spawn entities into the world but not changing actual geometry because that would require the game to constantly update and stream an updated version of the map to everyone every time someone makes a change, even if it's minor. 
I meant to say 'everything possible'. However, Changing the geometry 
is possible. Second life did it, and so did a game called Cube 2 
http://sauerbraten.org/ [Quote from: zookuw on December 11, 2010, 10:18:54 AM]
I have researched on the internet extensively and found projects that have accomplished the tasks that I aspire to do, as they were my inpiration. For example, Eskil Steenburg -- who made the MMO, "Love" -- also created a developer's tool called Quel Solaar. Here is an excellent video on it: 
http://www.youtube.com/watch?v=NBGz723H0_AThere is also HeroCloud. Here is a link to an overview (the first video is the one that explains what I am going to accomplish): 
http://www.heroengine.com/features/heroblade-all-in-one-development-environment/I read somewhere that the entire game of Second life with all the accounts, and on each account all their items that they made equals to about something like 10 Petabytes or something. So you would need A lot of storage. The reason being why this isn't as insanely popular is because of the storage issue. 
Second Life is an MMO, where every player connects to a master server hosted by the company who made the game. The project I am talking about will be more like Blockland; players set up their own servers and others join them.