When you're a leader of a Guild, you can join the bidding on the map by clicking the "Territory War" button in the Guild menu(see pic below). There you can see the territories available, and you can bid using the Contribution from your team! The top bidding 3 guilds get to join the war.
Here is a screenshot of where you can find the details. As it notes, the Guild Leader must reach certain levels to be able to bid for specific maps.