将一个用户添加到用户组中，千万不能直接用：

usermod -G groupA user

这样做会使你离开其他用户组，仅仅做为用户组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