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