How did roxas join organization 13

When Sora sacrificed his heart to save Kairi in Hollow Bastion, Sora's nobody (Roxas) instantly came into existence. Roxas woke up in Twilight Town with no recollection of his past. Xemnes came to him and gave him his name. Xemnas then brought him back to Castle Oblivion and made him an official member of Organization XIII (13).
Xemnas wanted Roxas in the Organization so that he could utilize the power of the keyblade to gather hearts so that they may make Kingdom Hearts complete and become whole again. :)