In the following menu, press the Change Union button. A window will appear where you can select one of the suggested communities, join a random union, or search for a union by its name.
When you choose the union you like, press the button with the crown icon and go to the union-viewing window.
The green Join Now button means that you can join the selected union right away.
You can be part of only one union at a time. You are assigned to your first union automatically. There’s a way to switch from one union to another, however. Open the main union window by selecting the Island on the map, and press the button with the gear icon.

In order to join some unions, you need to have reached a certain level, which will be shown by the level icon on the button.
Please note that you can only change your union once every 20 hours. Make your decision carefully!
Note that all points you have earned in the previous union, will be reset after you join another union!