To add Yuffie Kisaragi to yor party in Final Fantasy VII - you have to find her first. She is A Ninja hailing from Wutai, Yuffie, is only seen in Forests across the world, for an easier find, search the forest area near the Large Condor. In that forest Continue to engage in battles, Until a "Ninja Girl" appears. Then engage her in battle and defeat her. After that a scene will appear and you will talk to Yuffie. Make Sure you answer the exact answers, or you will not get Yuffie and you will have to repeat the proccess all over again. Upon defeating her, you'll see a the scene and answere these questions with these responses.

Yuffie : "You spikey-headed jerk! One more time, let's go one more time!"
Answer : "Not interested"

Yuffie : "You're pretty scared of me, huh?!"
Answer : "......petrified"

Yuffie : "I'm really goona leave! REALLY!"
Answer : "Wait a second"

Yuffie : "You want me to go with you?"
Answer : "......Thats right"

Yuffie : "All right! I'll go with you!"
Answer : ".......Let's hurry on."