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You know the drill. Reply to someone whose Friend Code you want, and be sure to include your own in your post.
Just FYI, because it doesn't seem to be common knowledge: Aside from adding each other's Friend Code, it is also important to connect online in Festival Plaza frequently. This is because the Battle Agency was designed by Hitler, and after 3 days of not being connected online at the same time with one of your friends, you will no longer be able to use them on your team.
So in short, register friends, hop online frequently, and don't forget to VIP your best battle partners.
Also, the Battle Agency gives you an absolutely insane amount of Festival Coins as you level it up. At the time of writing this, my Festival Plaza is Rank 411 and I have amassed nearly 170,000 FC. The general "Festival Plaza" buff given by the Fortune Teller Tents applies to the Battle Agency as well, so fill up your Plaza with FT Tents and try to battle most on days you get the best buff. You will start to get thousands of FC for each win.
My current Rank is 46. I'll probably hit 50 today. My Friend List is getting full, but I'll try to add everyone. Hopefully this helps those of you just starting out.