You connect to your friend's White Forest by both turning on the C-Gear first of all.
Than you go to Entralink, and go to one of the two bridges, and connect to the other player's world.
You are now in your friends game.
On the bottom screen, tap the location of Black City/White Forest, and you'll warp there.
Afterwards, walk around and talk to people, and maybe people may go to your world, maybe not.
Just walk around and talk to people. ~