I believe 'join the nightmare mode fans' means play on the hardest difficulty level, and possibly discus various tips and strategies you use (in your own topic or in response to problem areas other people post about), or just read about what others have done (though you would have to be careful about spoilers, if you have not finished the game on your own yet).
There are several older topics where people described their gameplay on nightmare difficulty mode, possibly with other restrictions to stats, skills or equipment (etc) to make things more challenging.
Who's playing DKS on nightmare difficulty ? (spoilers)
Nightmare mode on Xbox of a female hero (spoilers)
