There's a good number of North American players. Just log onto the Diplomacy server and you'll find people. There are normally several 4v4 and 5v5 games each afternoon and evening. (However, some people might be playing the Rebellion Beta now.) Do note that the original (Vanilla) Sins and Entrenchment expansion are on a separate server from the Diplomacy expansion, so you won't see all of the people who logged on for Diplomacy. Vanilla and Entrenchment activity is pretty dead, limited to insular groups of friends playing comp stomps.
