On CentOS, I need to grant a user root privilege, which was created before.
The usermod command modifies the system account files2 to reflect the changes that are specified on the command line.
Add the user to the supplementary group(s). Use only with the -G option.
-G, --groups GROUP1[,GROUP2,...[,GROUPN]]]
A list of supplementary groups which the user is also a member of. Each group is separated from the next by a comma, with no intervening whitespace.
usemod -a -G wheel <user name>would grant a specified user root priviledge.
From man page.
What files are system account files?