OMG gang quests should scale with number of gang members because there are many gangs that are filled with hundreds of people. Lets say the quest is to farm 5k wood, then with 2 people it scales up to needing 8k wood, then with 3 10k wood. This keeps people from just having literal randoms slave quests for them, and also incentivizes people to not have gangs full of inactive players.
For quest rewards you could have a perk that gives every gang member a 1.5x xp buff for 24 hours or a week.