There is an option for the 6th person at the table to referee the adventure, read the story moments and control the creatures if they choose to. Alternatively, the stories are written to balance out for up to 5 characters. There is nothing to stop a group from adding a sixth but it will require some house rule adjustments (ie: add 10 Vita and 5 defense to all creatures or add 1 additional creature to the combat setting, and in the case of Skirmishes, add +1 to the skirmish power or something along those lines, if no one wishes to read ahead in the story to adjust the difficulty)
Folklore was created to give everyone a chance to play in a moment's notice, with very little set up and virtually no preparation, sit all together as players and have an RPG style experience without one of the players ALWAYS having to learn all of the rules no one else knows, prepare everything ahead of time and run the game during every session. That being said, again, it is also designed so that someone could easily assume the role of GM without such a painstaking preparation, yet if anyone wishes they can create their own adventures rather easily and run them for other players.