Working on EC2 is a snap, you just download the .pem file, give it the right permissions, and you are ready to go, yet ... if you have the .pem file you have full access to the EC2 instance!!
What shall I do to limit people's access to the instance in a controllable way, e.g. people pass me their public key and add it to the instance a la Github
Follow the steps in this document: http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/managing-users.html
There are 3 steps: