Similar to the Battle Tree, you will get to use a team of three Pokemon to fight in the Battle Agency. However, you yourself only get to pick one! You can pick from three randomized Pokemon that will frequently change. You can see which three you can pick by seeing the Pokemon sprites floating around in the screen behind the Battle Agency receptionist.
Even when picking the same Pokemon the movesets are highly variable to change. There are literally thousands of different Pokemon and movesets that are available!
So where do the other two Pokemon come from? Your friends, of course! It pays to be social, doesn’t it? You can pick from guests who have visited in the last 48 hours or VIPs. As you scroll through your list of available allies, you will see they each have preset Pokemon. You can’t change this, so make as many friends as possible so you can build a diverse team.
This also affects the Pokemon that shows up to your friends. If you do not have friends who play at the Battle Agency often, you will find that you are at a level disadvantage, and it can be either difficult or outright impossible to win.
If you manage to join a group of friends who play Battle Agency, make sure you keep up with them so that your Pokemon won’t fall behind in level and ultimately be useless. And if you can find a friend who is at max grade while you are still working up to there, enjoy the level advantage while you can.