She's always part of the party, just not officially. Since the others knew she would always follow them around and could take care of herself, they never really cared if she was a few steps behind. They knew full well she would catch up, like a very determined puppy. Plus, she always helped them out without asking so they just accepted it. No reason to refuse.
She's basically the team pet that suddenly appeared for no reason and is just there by the background.