Why not reward those who go to war then? Boost production, morale, and control as your armies win more battles maybe?
As for the new player experience, I think there isn't much you can do about it.
At the end of the game, BM is a social game. Most people don't have the patience to wait a day to see a welcome message from another player. Not sure sending an automated welcome message will even do anything. Maybe sending a message where the game encourages people to introduce themselves and get involved right away wouldn't hurt.
