I have a suggestion of how could be implemented easily a way from anothers parties join a fighting already in curse, the player fighting or not.
When the battle starts, nearby parties who could have a interest in the fighting (belongs or is allied to one of the figiting parties) have a chance (in %) of join the fight as reinforcement. This chance should be greater at daylight, for obvious reason.
While the player is roaming the world map, another parties should join fights as well, but only if there is a hope to win. Maybe should be a limit of how many parties can join too ( one allied per side is fine).
When the battle starts, nearby parties who could have a interest in the fighting (belongs or is allied to one of the figiting parties) have a chance (in %) of join the fight as reinforcement. This chance should be greater at daylight, for obvious reason.
While the player is roaming the world map, another parties should join fights as well, but only if there is a hope to win. Maybe should be a limit of how many parties can join too ( one allied per side is fine).