Centos 7 - adding a user to sudoers group - still is not in the sudoers file - why?

[user@localhost ~]$ su -
Password: 
Last login: ...
[root@localhost ~]# usermod -aG wheel user
[root@localhost ~]# exit
logout
[user@localhost ~]$ sudo echo 123
[sudo] password for user: 
user is not in the sudoers file.  This incident will be reported.

What do I do wrong?

My experience is that 'user' needs to log out and in again. Try the 'id' command to see if the system thinks that 'user' is in the wheel group or not.

In stock CentOS 7, wheel is not enabled in the default /etc/sudoers file.

## Allows people in group wheel to run all commands
# %wheel        ALL=(ALL)       ALL

## Same thing without a password
# %wheel        ALL=(ALL)       NOPASSWD: ALL

Did you uncomment those lines?