I think people would join because there would be the reward of fame when you help towns and that is an important mechanic of the game. It also would be a good way to hone your skills in combat. There will be Daemons in the game the Dev's said and they are basically the decedents of Demons so the order could be dedicated to rooting them out and defeating them; however I think those threats are quite rare. I feel like you might end up killing more bandits than the spawn of evil just in order to keep yourself feed and equipped. As for a potion that would leave a visual identifying effect I am not sure but you could always have a guild cloak that members wear that identifies them.

We've kinda stuck to this but we now have a Troop tree built and the core concept is to protect our traders and Village also small villages that request our help but any Lords that request us will have to pay in gold, whereas village elders only have to give us a little food and whatever they want to offer (but we never ask)