Plaguescarred
Hero
From time to time i have an NPC work with the party. (I picked 25%)
The DM doesn't have the power to add an NPC to the group.
There are two ways this happens in my games:
1) NPCs are created by the players and are basically henchman for the party and possible backup characters in case of death. I always allow this if the player fears that much for their character.
2) When the NPC is important for the story. Example: The person that hired the party to help him find X, or to make sure they keep on track, or even guide them through lands they do not know.