The new Mac OS X is a BSD based user-friendly operating system made by Apple. Most of the authorization data are stored in what Apple calles the Net Info Database. It is a kind of LDAP tree in which all kinds of data are stored, like the Windows registy but much cleaner ;). All UNIX files like /etc/passwd, /etc/group and so on are only generated from the NetInfo and are not really important anymore.
Fun ends when you try to fiddle around with your new-born Mac and break the Netinfo Database. Some nasty things will happen:
- Your Control Panel will not respond to you clicking around in the Useres pane.
- won't connect to your local Netinfo.
- Lines like Apr 18 19:38:08 localhost lookupd[228]: dsstore_fetch_internal 80 Store.192 _dsstore_dsrecord_fread failed at offset %s may appear in your /var/log/netinfo.log

I even killed my admin group, which lead to a Machine without any administrative user on it. D'ough!

Surfing the web helped a bit and I would like to share the knowledge I gained in my trip around the os.
If you see above messages in the log, try to kill the lookupd in order to get the machine in a clean state using killall lookupd. If it doesn't go away you might have to "killall -9 lookupd" it. Now try to restart it using "lookupd". Now watch to log if it starts up cleanly. If it doesn't you should shut down the Computer asap.
The easiest is to fix it using another Mac OS Computer with the same OS Release (I think it just needs to be the same major revision, e.g. 10.3). To do that, put the broken one into target-disk mode (it will cause your mac to behave like a Firewire disk. Just swich it on and hold down the T key until the screen turns blue and shows the Firewire logo. Now plug in a Firewire cable from your rescue computer. The other Macintosh HD will show up in Finder.