To add a New User and Assign a Group on CentOS 6 terminal
#useradd -g <groupname> username
For instance, lets say you wanted to add a new user named user to the apache group:
#useradd -G apache user
And then you’ll want to assign a password for that user, of course:
passwd user
Add or update User to an an Group
#usermod -a -G <groupname> username
Change a User Primary Group
Sometimes a user has many group, this is switch out the primary group that a user is assigned to with this command:
#usermod -g <groupname> username
View a User Group Assignments by use the id command to see what groups the user is assigned to:
id <username>
This will display output something like this:
uid=500(howtogeek) gid=500(howtogeek) groups=500(howtogeek), 1093(admins)