Coalitions are temporary pacts within one game round. If you want to collaborate with other players you can create or join a coalition. To do that access the coalition menu in the upper left corner.
First and foremost coalition members can win the game together and achieve shared victory rewards. Additionally coalition members can communicate effectively with an own coalition chat as well as a shared diplomatic conversation. Besides that, when coalition members take back core provinces from an enemy they automatically trade these core provinces back to the coalition members who originally owned them.
Create a coalition
To create a coalition you just have to click on the "create a new coalition" button. Afterwards you can enter a name, description, and upload a coalition flag. Make sure the picture does not violate any rights and is not too big, otherwise you cannot upload it. Once you have created a coalition, other players can apply for it. As coalition leader you can accept or decline applications as well as kick or promote members in the coalition menu.
Join a coalition
To join an existing coalition you have to open the coalition menu and click on the info button next to the coalition you want to join. Inside the following menu, click on the "apply" button. The coalition leader will then receive your application.
Leave a coalition
To leave a coalition, just click on the "leave coalition" button inside the "my coalition" tab.