NPCs come in a variety of flavours - those being run by Marshals and those being played by anyone else. Usually they perform a generic role within a game scenario. From VIP to be escorted to the cultists needing to be defeated.
This is pretty much the default for warbands, especially those that contain a majority of players not interested in roleplaying. They're willing to work for any side, for the sake of “payment”.
For this to work, a mercenary group (or individual) must have standard rates of pay, including if they work for more than one side in a scenario, and what conditions they are able to break their contract.
Unless otherwise noted, as a villager or peasant, players can't be exotic classes and should use equipment loadouts that aren't too unreasonable.