To add ssh key access to users on Amazon EC2 follow these steps
Note: upload user public key file in .ssh/id_rsa.pub
1. Add New User: useradd -c "Mohit Sharma" mohit
2. Navigate to user directory: cd /home/mohit
3. Create .ssh directory: mkdir .ssh
4. Give Permission: chmod 700 .ssh
5. Set Diretory Group and Onwer Permission: chown user:user .ssh
6. Write uploaded key to authorized_keys: cat id_rsa.pub >> .ssh/authorized_keys
7. Give Permission: chmod 600 .ssh/authorized_keys
8. Set Group Owner Permision on keys: chown user:user .ssh/authorized_keys