Right now how it works is that those willing to join will just join you and adopt a similar, but darker color to the color of your character. Those that need to be forced would be those that start shooting at you. Generally speaking, someone who is able to attack (by unlocking the volley skill for themselves), or has more followers than you won't willingly join your ranks, so wittingly down their numbers till they either can't attack or they decide you have too many will coerce them to join.
You are right though, that I didn't make it clear, it didn't occur to me until you mentioned it, so thanks for bringing it up!