You can play either over a LAN or over the Internet on the MS Zone. Finding people to set up a game with is quite easy also since the Zone will use its matchmaker system for easy searches and there's also a staging area for chatting and setting up games before and after a session.
The first style of multiplay game simply runs a group of players through the single-player campaign -- they can work together as cooperatively or as competitively as they see fit.
The final mode of multiplay, will take place in a H U G E world even larger than the normal single player world. Player again can work individually or cooperatively to conquer this vast multiplayer world.
OK, so now you've got a game picked out and you're ready to get in there and get your hands ready. You'll need to be able to differentiate your team from others. In multi-player mode you can choose from a variety of base clothing colors and styles which will allow characters to be easily identified. You can have a party up to a total of eight in size. The number was previously ten. Chris explained the reasons to the DS Vault:
[Ten] was a number that we planned on supporting when we started the project. Our current thinking is actually to reduce this number to eight players. Our tests have indicated that the host computer would need to be very powerful to support ten players, which is why we are reducing this number.
As far as the actual mechanics of getting everything to work, GPG has indicated that a player without the files for the specific map/scenario will be able to download them from the host. The host will have an option to allow or disallow file transfers.
You won't have to worry about losing hours of game advancement in a multiplayer game either since GPG allows for saving a multiplayer game by the host:
"The Host can save the game just like they can in single player. Anyone can show up for the session when the saved game is loaded, new players and players from the actual session. Characters are saved separate from the world, so this is entirely up to the individual and how they want to manage the multiplayer sessions." - Chris Taylor interview at the DS Vault.