This is a weird problem. It occurred for me when I was trying to login as another user from root. For example,
[root]# su user77
It generated the below error.
su: /bin/bash: Permission denied
I have seen many geeks from forums suggested several methods to resolve this issue. I have tried most of those. So not sure which one worked for me. But I think this one worked for me, so it can work for you too.
gpasswd -a username wheel
If you are still having this problem, then dig more at google.