Amazon AWS EC2 instances are by default secured with ssh certificates. This is great for security until you need to provide a UAT duplicate for an external user or developer. You don't want to share your certificate with them and setting up a new one is more work than this quick fix. The security isn't as important on a UAT or test system as it is on a production system so for this use case we can go for lower security.
To enable users to access we first need to set a password on the ec2-user account. It's highly recommended that you select a strong password!
sudo passwd ec2-user
Then we need to allow password only connections. We edit the ssh settings, find the line PasswordAuthentication no and change it to PasswordAuthentication yes.
sudo nano /etc/ssh/sshd_config
Then we need to restart the ssh service.
sudo service sshd restart
Now you can login to you Amazon AWS EC2 instance with only a password. To secure the server again just change the PasswordAuthentication line back to no.