There are 4v4 and 5v5 games on the Diplomacy expansion every day. Most of the action is during the European and North American evenings, say from 3 pm - 11 pm U.S. Eastern Time. Also, there's more activity on the weekends. You do have to wait for games to open and fill up, however. (Once you start your first 4v4 or 5v5, you'll be "in sync" with other players and probably won't have too wait to long for the next 4v4 or 5v5 after your current game ends.)
The online multiplayer rocks once you get up to speed, and now is a good time to get into it since you'll be ready when the Rebellion expansion is released and (hopefully) scores of new players log on.
If you join a game and receive this error message, "Your Mesh Files Differ from the Host" and it doesn't look like a game with a mod, then you may need to fix the Mesh File problem, which isn't hard to do. Visit this thread for details: http://TinyURL.com/Sins10