if a user is using an invalid shell according to /etc/shells
that means that whatever shell they are using is not listen in /etc/shells
so for example if the user is using bash, then in /etc/shells you want to make sure that the line
/bin/bash
exists. make sure you have the full path to the shell, or else it wont work. you cant just put in 'bash' on a line in /etc/shells , you will still get complaints of invalid shell. a user using an invalid shell can still login from the console, but cant su, and cant login from anywhere else.