Players can form an Alliance to chat with each other and share their strategies. To join an Alliance you need to reach Warlord Level 5 and liberate the Alliance HQ on the map.
Once it is unlocked, you can tap on it to open the Alliance menu. If you are not a member of any Alliance, you can choose to join an existing one from provided list. You can also search for a specific Alliance.
Have a look at the right side when browsing Alliances, some of them will be open and you can become a member immediately without sending a request. Other Alliances will require manual approval by their Alliance Leader.
Note: You can only be member of a single Alliance at a time. If you want to join another Alliance, you have to leave your current one first.