Is a girl considered an infidel for joining the Ba'ath Party in her youth without knowing that it was disbelief?

As long as the woman is ignorant of the party's principles and what it advocates, her acts of worship and obedience are valid and acceptable. No one is judged to be a disbeliever until proof is established against them and the path of misguidance becomes clear to them, for Allah Almighty says: "And never would Allah lead a people astray after He has guided them until He makes clear to them what they should avoid."