Follow this article.
1- Launch the Settings app from your Home screen
2- Scroll down and tap on Touch ID & Passcode
3- Enter your passcode if prompted.
4- Under the Fingerprints section, tap Add a Fingerprint...
This will bring you through the Touch ID fingerprint registration process. When you're done, repeat again for any additional fingerprints you want to register.
Don't forget to share this awesome key to your friends by clicking sharing buttons below.