Not Authorized on debian testing + xfce4.8

[chengshid <chengshid@gmail.com>]

my OS : debian testing + xfce4.8
There are some problem when i as a no root acount.
1. When I  plug a removable disk in my computer,  it is come out a ERRORDIALOG with the Not
Authorized.

2. i is not  authorized to shutdown and reboot the computer.

i have done some work as follows:
 i have added myself to  the "sudo" group.
 i konw i should add myself to the system groups example "power" group or "plugdev" group or "shudown" groups. But, i can't find the groups in the /etc/group.


$ cat /etc/group
root:x:0:
daemon:x:1:
bin:x:2:
sys:x:3:
adm:x:4:user
tty:x:5:
disk:x:6:
lp:x:7:
mail:x:8:
news:x:9:
uucp:x:10:
man:x:12:
proxy:x:13:
kmem:x:15:
dialout:x:20:
fax:x:21:
voice:x:22:
cdrom:x:24:user
floppy:x:25:user
tape:x:26:
sudo:x:27:
audio:x:29:user
dip:x:30:user
www-data:x:33:
backup:x:34:
operator:x:37:
list:x:38:
irc:x:39:
src:x:40:
gnats:x:41:
shadow:x:42:
utmp:x:43:
video:x:44:user
sasl:x:45:
plugdev:x:46:user
staff:x:50:
games:x:60:
users:x:100:
nogroup:x:65534:
libuuid:x:101:
crontab:x:102:
Debian-exim:x:103:
mlocate:x:104:
ssh:x:105:
user:x:1000:
fuse:x:106:
messagebus:x:107:
utempter:x:108:

What's happened with my debian ?
thanks!