I have been working with SSH for a long time, but recently I have needed to add another user to my set up. I followed some instructions I found on the web and did the folllowing.
adduser sam sudo nano /etc/ssh/sshd_config AllowUsers michael, sam sudo reboot
I set up the user sam completely, added the allow users to the sshd configuration and rebooted. Now michael is sudoer, but the funny thing is, I can login via ssh with Sams credentials but not michaels anymore.