They just chain mobs for the NM spawns, aka bears or one of the other nm spawns, basically think of it as a hunt without the going to check if it's up and forcing it to spawn.
You won't get much xp or loot from the higher ones, they act like overworld fates but spawn based off mob kills and a time.
Forming a party imo is easier at times other times it's easier just to pop in and ask for an invite, depends on the time a day and all that.