You can earn this badge by participating in the 39C3 LED badge soldering workshop and visiting the TBD assembly for a code. The 39C3 LED badge reacts to the presence of friends in its vicinity, which includes the TBD friends badges.

There is also a completely different badge (39C3 friends' friends badge badge) that you also can earn at the TBD assembly by showing that you have a 39C3 friends badge gifted from a friend.

TBD friends badges also count as a 39C3 friends' friends badge