How do I join my friend's alliance when they are in a different area?

Do you mean somewhere else in your province (state) or in another province? You should be able to join any alliance you want. Go to the alliance menu (if you are not currently in one) and you will be able to search for alliances to join.

If it is in a different province, I believe you can still join alliances that are in another province but it will be difficult for you to receive all the benefits of being in that alliance unless you move to the same province as it. If your game is below level 6 then you can do this for free. If not then you may be better off starting a new game and moving it the the province of your friends alliance.

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