Why can't I create directories under /root as a member of root?

Question

I've done the following:

sudo usermod -a -G root userA
sudo su - root
cd /
chmod 775 root
ls -aln
-- drwxrwxr-x.   6 0 0 4096 Sep 13 22:04 root

I can cd /root from my userA, but I can't make directories.

mkdir testing
mkdir: cannot create directory ‘testing’: Permission denied

For completeness:

id userA
uid=624981(userA) gid=624981(userA) groups=624981(userA),10(wheel),0(root)

What am I missing?

Answer 1

In order for the addition of the user to the group to take effect, you have to logout and relogin.