NPC-driven factions are factions that players can join and can influence for example player can become NCR president. Membership in an NPC-driven faction allows you to access areas and complete quests unavailable to non-members. Joining/leaving an NPC-driven faction will affect your reputation with various NPC groups. Every NPC Faction member has to respect its codex. If you want to join one its good to ask on official Fonline3 discord server.