Ash has agreed to let anyone join him for reasons unknown. From girls with grudges over bikes to newbie trainers, gym leaders, younger siblings, and dragon girls. Who hasn't he let join him.
He even invited Meowth from Team Rocket to accompany him on his journey when he found him unconscious and injured. Of course this was all a trick to gain Ash's trust so he could steel Pikachu and Ash's other Pokémon.
Source: Watched every episode of the Anime.
Hope I helped :)
No one actually. I guess he doesn't mind having a few people with him.