No option to join factions

[Ferrouswheel]

 

I'm in the middle of the 17th quest, and it says to progress I need to join a faction. I don't have any options to join factions, and I watched a video of someone completing the quest, and the two faction people appeared in front of them here. Any help?

 

Edit - I never got a letter offering to join the different factions early on, how do I get it?

[Nutrient]

The quest to join a faction is in the Bamboo Village. It'll be one of the blue ones!