@Cursed_Spirit: The topic maker is asking about recruitment chance, not the place where you find it. And FYI, Shaymin and Arceus are not coded in this game because they are yet to announce when this game is out. Drayano wrote in Legendary recruitment guide
and with some of my correction based on a reliable source. Cresselia, Phione, Manaphy: Auto-join
Dialga, Mesprit, Uxie, Azelf: 100%, after credit rolls.
Regigigas, Palkia: 100%, 2nd time.
Ariticuno, Moltres, Zapdos, Lugia, Ho-oh, Celebi, Mew: Easier (10-20%)
Regirock, Registeel, Regice, Suicune, Raikou, Entei, Heatran, Giratina: Easy (5-8%)
Latias, Latios: Medium (0.1%)
Deoxys, Groudon, Rayquaza, Kyogre: Hard (-10%)
Jirachi: Harder (-12%)
Mewtwo, Darkrai: Very Hard (-22%)
That list above is only for your reference. The experience in-game may be different and at some point, it contradicts with the original guide.