you have to beat the game first. When you beat the game, you are now able to recruit the legendary pokemon. You just have to go to the dungeon where you found them, and fight them again. All the legendary birds have a 33.33% recruit rate. Groudon has a -10%. Kecleon has a -33.9%(I think, you have to be Lv.90+ with a friend bow to recruit a Kecleon(which is Lv.90 itself(don't even think about shoplifting!)))The higher level you are, the higher the recruit rate goes. If you are holding a friend bow, it goes up by 10% more.
More info here.
I played the game.