I think is not necessary server side send new information to client, you could do this all through the client side.
You will need to do the same as the battle window to get the players on your game screen.
Then, you need just to check, from these players, who has the shield icon of party.
So, it's possible to do all this system through the client side.
Anyway, seems easy, but it's a lot of work in my opinion.