Yes, you can send mass messages on atwar, instead of going player-to-player.
First, go to your messages, and in the top right (below all the tabs to different area's of AtWar), there is a "New Message" tab. Click that.
Next, it will bring you to a new page where you can type in names of players in 1 box. after each player's name, put a comma (",") with a space in front of it and you can add the next player's name (example shown in the box below). This list can be unlimited.
And that's how you send out a mass message. Don't abuse it!