Is my group file normal?

[Cheeseboy]

EDIT: Changed the topic to reflect the question.

Hi all,

I'm unable to use sudo on an Ubuntu machine that uses the bubba as NIS server for authentication.
The authentication seems to be successful. I get no error messages at the client or in auth.log on bubba. But the command followed by sudo never executes:

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niklas@desktop:~$ sudo ls
[sudo] password for niklas:
niklas@desktop:~$

I have no idea what change I made to introduce this problem, it probably took me a while to notice it. It could have been an Ubuntu upgrade, Bubba upgrade, or just executing make in /var/ip on the bubba. Or something else... Note that this is something that used to work.

Looking at ubuntu forums it seems this can happen if the user is not part of the admin group.
Having a look at the /etc/group file on the bubba I notice that there is no admin group, and I also notice that there are actually no users belonging to any group. Is this normal? How does the group resolution work on the bubba?

/etc/group:

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root:x:0:
daemon:x:1:
bin:x:2:
sys:x:3:
adm:x:4:
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:
floppy:x:25:
tape:x:26:
sudo:x:27:
audio:x:29:
dip:x:30:
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:
sasl:x:45:
plugdev:x:46:
staff:x:50:
games:x:60:
users:x:100:
nogroup:x:65534:
postfix:x:101:
postdrop:x:102:
crontab:x:103:
ssh:x:104:
dovecot:x:105:
lpadmin:x:106:
messagebus:x:107:
avahi:x:108:

If I create a new file on bubba it will be listed as group "users":

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niklas@bubba:~$ touch test
niklas@bubba:~$ ls -l test
-rw-rw-rw-  1 niklas users 0 Jun  6 17:10 test

Where does it get the "users" group from? No members are listed in the group file...

Am I barking up the wrong tree here?
If my groups file is normal, then the problem must lie elsewhere.

I tried to be clever:

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root@desktop:/home/niklas# strace -f -o strace.out sudo ls

But I was unable to spot anything strange in the enormous output. And I'm not even sure if sudo would behave the same when executed by root (as I had to do to get strace working)

Thankful for any insight,

Cheeseboy