출처: http://www.cyberciti.biz/faq/howto-linux-add-user-to-group/ H ow can I add a user to a group under Linux operating system using command line options? You can use the useradd or usermod commands to add a user to a group. The useradd command creates a new user or update default new user information. The usermod command modifies a user account and it is useful to add user to existing groups. There are two types of groups under Linux operating systems: Tutorial details Difficulty Intermediate ( rss ) Root privileges Yes Requirements usermod/useradd Estimated completion time 5 minutes Primary user group. Secondary or supplementary user group. All user account related information are stored in the following files: /etc/passwd - Contains one line for each user account. /etc/shadow - Contains the password information in encrypted formatfor the system's accounts and optional account aging information. /etc/group - Defines the groups on the system.