I created a new user on my AWS EC2 RHEL 7 instance that I just use for learning purposes. I also created a new SSH key for use on this server. Everything looked to be configured for the user in terms of having created the ~/.ssh/authorized_keys file on the server with the public key. I set the permissions correctly, but I was still having trouble logging in as I kept getting a permission denied message (Permission denied (publickey,gssapi-keyex,gssapi-with-mic).

I checked /etc/ssh/sshd_config to see what the setup looked like and I didn't see any explicit declarations for AllowGroups or AllowUser. But on a whim, I just tried adding the newly created user to the 'adm' group which the default 'ec2-user' was a part of and everything worked as expected.

So my question is, if not in the /etc/ssh/sshd_config file, where is the configuration specified to allow members of the 'adm' group to login via ssh?

While I can't answer the question, I would use verbose options (ssh -vvv) to better understand what's going on, and check /var/log/secure, where successful and failed login attempts are logged. The sshd log might also help (journalctl -u sshd).

And, what is getting logged in /var/log/secure or /var/log/messages when you try to login as this new user?