You can create or join a clan from the Clan window, which you can open from the menu by clicking the shield icon. Creating a clan is not free but if they can do it so can you. Alternatively you can join an existing clan providing you find one that you like.
Depending on your rank in a clan you may have access to the clan vault, which is similar to your personal bank, but is accessible to those that have a sufficient rank to use it. The clan leader (SupremeGeneral) can set the required ranks along with other various clan related settings.
Clan needs 10 members to initiate a siege against a hamlet and 20 members to initiate a siege against a city.
When initiating a siege the attackers need to place a siege stone near the holding. The maximum distance is 1001 meters from the holding that is going to be under siege.
Leaving a clan is a long transaction as it takes 45 minutes to fully leave the clan.