Each can be set on a team depending on how you want to set alliances, and for hosting there are a large number of tweaks that can be made to how a match plays out.
In the preview build we have access to, which keep in mind is not a complete version of the game, there's room to invite up to 16 players into a match, and it appears as though you can mix AI opponents with human players. I had the chance to get a look at how the game performs with others, and though it's difficult to make any sort of judgment call after only a few hours of play, it seems to be something Civ fans are going to like.
From what's been seen so far, there's little doubt that Civilization V is going to be a lot of fun when it's released on September 21, but what happens when you get tired of playing against only the AI? Well, then it's time to dive into the multiplayer mode and add one or more friends.