I'm going to build off of my previous suggestion to make enemy guilds red indicators. When you 'declare war' on someone, it sends the leader of the other guild a notification to accept. Once they accept, it sort of becomes a little mini game. Kill the enemy guild members on sight. Your goal becomes to kill everyone on the enemy team. Once you kill one, their indicator disappears and they're no longer in the war. They can still kill you, but it wouldn't count. If you are killed, you're out of the war and your kills don't count towards them. You can still help your guild try to catch the remaining enemy members through.
To win the war, you must either eliminate every opponent on the enemy team, or if a set amount of time expires, the team with the most surviving members wins by default, this way you can't just hide it out and hope to win. To the victor goes the spoils. The winning team gets 100 gralats for each member on the losing team. From their accounts.
Everyone who is in the guild when the war begins are the ones who can participate and receive money. If you recruit someone after you start a war, they're not in the war. If you kick someone from the guild who was in the war, they're out. If they aren't active on that guild tag or have been inactive in general for 'x' amount of time, they're out of the war. Any kills off tag don't count. I'd say to be fair, enemy guilds should be limited to 5, as there are 5 ally guilds.