centos将一个用户添加到用户组中

usermod -G groupA username

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