There're two parameters you must use when launching a network game: ‘-dis-site’ and ‘-team’. The first one sets the player ID for this session (and must be unique between all the players), while the second selects a team (all players of a team start the game in the same point of the map).
Let's suppose you want to play a game with a friend. You could run ACM with this arguments:
acm -dis-site 1 -team 1
While your friend should use a different ‘-dis-site’ and ‘-team’ (though this one can be same, seems pretty pointless that both players start the game in the same point of the map):
acm -dis-site 2 -team 2