If you want Articuno to join your team you have to have beaten the game and you I know have tryed to get Zapdos or Moltres before you would try to get Articuno but you can get Articuno to join your team after you beat Zapdos 9 times and you beat Moltres 5 times then go to Frosty Forest and go to the peak and battle him if you win(as I know you would win)he will join your team(I did this at a gold rank and I got him and you will have more luck if you are gold rank or higher)