Diplomacy
IS allowed in the game. As long as it's not secret diplomacy it's ok.
I can ask another player to help me beat another player in a game as long as I say it in the game chat. That's ok to do.
So, It's
not secret. It's just diplomacy.

This is posted on the homepage of CC :
Risk all your troops on a daring land grab.
Use diplomacy to coordinate a group assault on the game leader. Feel the thrill of victory as you crush your last opponent.