将一个用户添加到用户组中，千万不能直接用：

usermod -G groupA

这样做会使你离开其他用户组，仅仅做为 这个用户组 groupA 的成员。

应该用 加上 -a 选项：

usermod -a -G groupA user

(FC4: usermod -G groupA,groupB,groupC user)

-a 代表 append， 也就是 将自己添加到 用户组groupA 中，而不必离开 其他用户组。

命令的所有的选项，及其含义：

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 set password inactive after expiration to INACTIVE

-g, –gid GROUP force use GROUP as new primary group

-G, –groups GROUPS new list of supplementary GROUPS

-a, –append append the user to the supplemental GROUPS

                            mentioned by the -G option without removing 
                            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 (use only with -d) 

-o, –non-unique allow using duplicate (non-unique) UID

-p, –password PASSWORD use encrypted password for the new password

-s, –shell SHELL new login shell for the user account

-u, –uid UID new UID for the user account

-U, –unlock unlock the user account

查看用户所属的组使用命令：$ groups user

或者查看文件：$ cat /etc/group

Linux添加用户（user）到用户组（group）