NPC assist(also NPC allies, NPC crowd,faction guards) is generally the term for scripted or AI-initiated teaming of NPCs with player character to aid him with a quest or movement through a fixed location.
The inverse of "escort mission" quest, the player doesn't have responsibility for NPC safety and just needs to complete the required task/quest(being escorted by NPC guards/assistants).
This is different from hiring mercenaries as the action is scripted in advance and is scaled to amount of players at location: usually limiting NPC assist if the location is filled and increasing if the location is empty.
The psychological effect of "fighting in crowd"/"Safety of crowds" is very appealing to novice players and the NPC assist creates a deeper connection with player than NPC advice or mercenaries - however for experienced/veteran players the NPC might just be in the way. NPC assist works better on smaller/private servers.