Getting your Game Multiplayer compatible
To be able to play ETW games together, player must have the exactly same game-version. This means for IS Media.pack file contained in the main IS 2.1 download and the IS DB.pack and patch_is_text.pack from the Patch 2.1.3 download, both found first post of the download thread.
The files must be unmodified, the game compares the timestamps!
You must also load no other mods. In other words, your user.empire_script.txt must contain exactly the lines
mod "Imperial Splendour DB.pack";
mod "Imperial Splendour Media.pack";
and nothing else. Alternatively you can only have those two files checked by modmanager, in that loadorder.
Even though the localisation file (patch_is_text.pack) is not enabled via this script, it is still checked. You must use the standard localisation. Other localisations, e.g. the spanish and english one, or localisations from submods like AUM or the UCP may mess it up. Remove them from you data folder.
If you also want to play a Multiplayer campaign with IS starting units, you must copy the startpos.esf and scripting.lua from the campaigns/imperial_splendour folder into the campaigns/main folder. Backup the vanilla files!
Playing ETW online
You can play custom battles (up to 4 players) and even a campaign (2 players) online. The campaign is still in beta, you must sign up for the beta test in order to play it. ( http://www.totalwar.com/empire/beta/index.php ). Having done so, you can easily select Custom Battle or Campaign in the Multiplayer Menu.
You can chat during MP games by moving your mouse-cursos to the upper left corner of the screen, where the advisor would be. The Chatbox will appear and clicking the middle button will let you type.
During campaigns we noticed that sometimes the player can not do anything (all buttons greyed out). This can usually be solved by saving the campaigngame, and restarting from the save.