E esse mistério aqui:
marcos@ubuntu:~$ sudo adduser sandra
[sudo] password for marcos:
adduser: O grupo `sandra' já existe.
marcos@ubuntu:~$ userdel -r sandra
userdel : usuário sandra não existe
marcos@ubuntu:~$
o meu /etc/group :
root:x:0:
daemon:x:1:
bin:x:2:
sys:x:3:
adm:x:4:marcos,gabriel,gisela,luiza,familia
tty:x:5:
disk:x:6:
lp:x:7:marcos
mail:x:8:
news:x:9:
uucp:x:10:
man:x:12:
proxy:x:13:
kmem:x:15:
dialout:x:20:marcos,gabriel,gisela,luiza,familia
fax:x:21:gabriel,gisela,luiza,familia
voice:x:22:
cdrom:x:24:marcos,gabriel,gisela,luiza,familia
floppy:x:25:gabriel,gisela,luiza,familia
tape:x:26:gabriel,gisela,luiza,familia
sudo:x:27:
audio:x:29:pulse,gabriel,gisela,luiza,familia
dip:x:30:gabriel,gisela,luiza,familia
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:gabriel,gisela,luiza,familia
sasl:x:45:
plugdev:x:46:marcos,gabriel,gisela,luiza,familia
staff:x:50:
games:x:60:
users:x:100:
nogroup:x:65534:
libuuid:x:101:
syslog:x:102:
klog:x:103:
scanner:x:104:gabriel,gisela,luiza,familia
nvram:x:105:
fuse:x:106:gabriel,gisela,luiza,familia
ssl-cert:x:107:
lpadmin:x:108:marcos,gabriel
crontab:x:109:
mlocate:x:110:
ssh:x:111:
avahi-autoipd:x:112:
gdm:x:113:
netdev:x:114:
pulse:x:115:
pulse-access:x:116:
pulse-rt:x:117:
saned:x:118:
messagebus:x:119:
polkituser:x:120:
avahi:x:121:
haldaemon:x:122:
admin:x:123:marcos,gabriel
marcos:x:1000:
sambashare:x:124:marcos,gabriel
Debian-exim:x:125:
winbindd_priv:x:126:
vboxusers:x:127:
uml-net:x:128:
clamav:x:129:
familia:x:1006:
gabriel:x:1002:
sandra:x:1003:
gisela:x:1004:
luiza:x:1005: