Linux add Users (user) to user groups (group)

http://blog.csdn.net/pursue168/article/details/6443046

Add a user to a user group and never use it directly: Usermod-g Groupa
Doing so will allow you to leave the other user groups, just as members of this user group Groupa. Should be added with the-a option: usermod-a-G groupa User

(fc4:usermod-g GROUPA,GROUPB,GROUPC User)-a represents append, which is adding itself to the user group Groupa without having to leave another user group.
All the options of the command, and what it means: options:-C,--comment comment         new value of the GECOS field-d,--home home_dir           new home directory for the user ACCOUNT-E,--expiredate expire_date set account expiration DATE to Expire_date-f,--inactive inactive   & nbsp;   set password inactive after expiration                                   to Inactive-g,--gid group                force use GROUP as new primary group-g,--groups groups      & nbsp;    new List of supplementary groups-a,--append           Append the user to the supplemental GROUPS     &NBsp;                            mentioned by the-g option without removing    & nbsp;                             him/her from other groups-h,--help                     Display this Help message and Exit-l,--login new_login         NEW value of the login name-l ,--lock                     Lock the user account-m,--move-home                move contents of the home directory to the new                                  location (with only with-d)-O,--non-unique               allow using duplicate (Non-unique) uid-p,--password password& nbsp;      use encrypted password for the new password-s,--shell shell              New Login shell for the user account-u,--uid uid  & nbsp;              new UID for the user account-u,- -unlock                  Unlock the user account

To view the groups that the user belongs to using the command: $ groups User

or view files: $ cat/etc/group

Linux add Users (user) to user groups (group)