Whether you play on an alliance server or not, quick and clear communication with your allies will give you a major advantage. There are several ways to communicate with fellow players. Ideally, each type is suited specifically for certain needs.
The in-game messenger is a great way to send messages to people on the same server. Click the Messages link on the left-side game navigation bar. Enter the # of the player you want to send the message to, write and send the message. Alliance leaders have the ability to send messages to everyone in their alliance.
Earth Empires Forums
http://forums.earthempires.com/ are the official game forums. You will find the forums are broken down into specific topics. These are entirely public forums.
Alliance Hosting Sites
When you are part of an alliance, you will likely be asked to sign up at a hosting site. You will probably find many alliance-specific forums, and news and market monitoring tools. These forums are usually private to the alliance.