[fortepianissimo]

In short, in Tiger the group ID and user ID are the same. If your name is "foo", in Terminal if you type "ls -l" (sans the quotes) you'll see something like this:

...
drwxr-xr-x 6 foo foo 204 Oct 31 20:02 Desktop
...

Unfortunately Leopard now uses group "staff" for non-root users, so you should see this instead

...
drwxr-xr-x 6 foo staff 204 Oct 31 20:02 Desktop
...

The serious problem is that Finder doesn't know the group "foo" (the gid is not even in /etc/group). People have been reporting Finder crashes when trying to change file permission on any file with "(unknown)" group membership. You can find the detailed description and solution in this thread:

Apple - Support - Discussions - File permissions/users/sharing ...

[Hal Itosis]

Originally Posted by fortepianissimo 

In short, in Tiger the group ID and user ID are the same.
Unfortunately Leopard now uses group "staff" for non-root users.

 
Whoa whoa whoa. Hold on Nelly!. This is old news, ***in reverse***.
Cheetah, Puma and Jaguar used ``staff ' ' to group all users. Panther
did away with that scenario for the more secure paradigm (with each
user having their own group).

Originally Posted by fortepianissimo 

The serious problem is that Finder doesn't know the group "foo" (the gid is not even in /etc/group).

 
Well of course it wouldn't be in /etc/group. No user's group ever WAS there.
We had it in NetInfo's database... and now Leopard phased that out in favor
of Directory Services.

Originally Posted by fortepianissimo 

People have been reporting Finder crashes when trying to change file permission on any file with "(unknown)" group membership. You can find the detailed description and solution in this thread: <Apple Discussions>

 
Well, that kluge might bandage the Finder symptom... but it's a far cry from
being the "proper" fix. Somewhere along the upgrade path, these users didn't
get their NetInfo transferred into Directory Services. *THAT* is where the OS
[and Finder] will learn about humans and the groups they are members of.
[Not /etc/group]

Try this in Terminal:

dscl . -list /Groups PrimaryGroupID | awk '{ print $2,$1 }' | sort -n | pr -t -3 -w 108

If (human) user groups don't appear at the end of that list, then their settings
weren't carried over by the upgrade procedure. Mine sure were. (I backed up
to a disk that honored (not ignored) ownerships & permissions, erased the HD,
ran a clean install, and finished off by importing users using Migration Assistant).

-HI-

[Hal Itosis]

Now here's a post at Apple Discussions that looks to be on the right track:
Apple - Support - Discussions - Finder crashes when attempting to add ...

[I didn't try it, since I don't need to... but at least it *looks* right.]