Other players can invite you to their unions, even if you are the leader of your own union. To see the invitations, open the settings menu and press the Invitations button.

Note that all points you have earned while being in the union, will be reset after you join other union!
Every invitation contains the name of the sender, the date, as well as the union’s name, emblem and the amount of union points.
If you don’t want to be invited to other unions, just check the appropriate box at the bottom of the screen.
To accept an invitation to another union, you first need to transfer your leadership to another member of your current union. You can do that in the member management window.