Overcome /etc/nologin

Source

The file /etc/nologin locks your system down. However, sometimes you might still wanna leave people/systems in. Try extending PAM:

# /etc/pam.d/sshd
# Allogin login for members of the group 'admin'
+ account [success=1 default=ignore] pam_succeed_if.so quiet user ingroup admin
account required     pam_nologin.so