We think that is important for members of our community be able to play with each other online no matter where they buy their game. This is not the case right now. For example, if you buy Imperator: Rome from GOG, you can currently only play with other GOG customers or people who play with a Paradox Store purchase. Likewise, people who use a Steam account to play the game can only play with other Steam customers (or people who buy from the Paradox Store but redeem the code on Steam).
We are working on a solution to this tricky problem by building our own backend to handle multiplayer requests, but to unify the games under one system we need to have a common connection. To this end, we will be using your personal Paradox Accounts as the player identity. So, even though this system is not yet in place, we are asking you to log in to your Paradox account when you initiate a multiplayer game.
We are currently investigating if it is possible to have a viable multiplayer beta branch active in Imperator, like the one currently active for Stelllaris. Once we have more information, we will let you know.
In the meantime, if you want to conquer the Mediterranean with your friends, be sure that you all are buying from compatible online stores!
We appreciate the patience of our community, and hope to have a technical solution as soon as possible.