Warlight needs to make something special for diplomacy games.
First, They need to make separate rankings.
Second, They need to make a setting in the create games page.
Third, They need to add an 'alliance chat'. And the creator of the game can add people do it at requst of the other players.

What I have done for the alliance chat is to create a new game and invite the people who are in the alliance. I know it is not ideal, but it works.

I've done the same. For my game of thrones diplo games, I create a new game for the small council. It works, but it would be simpler if we had alliance chat

Yeah, because we always have to talk to someone who talks to someone else who talks to the group and it is really confusing.

Indeed, Diplos could do a lot for this game.