Access permission (was: New (GNU) kernels--what I think)

[Charlie Geyer]

In article <TALE.89Jun9202950 at imagine.pawl.rpi.edu> tale at pawl.rpi.edu writes:

> With MTS I can always permit my files exactly the way I want to and
> limit or give as much permission to the file as is my wont.  If I just
> want a particular account to have read access to a file, I can do it
> and not have to permit the entire project (group) access.  If I want a
> certain programme to be able to access the file, I can permit it so.
> If I want a whole project to have access, no problem.  And I don't
> have to go around making new groups for people to be in and setting
> GID or UID permissions; allowing the programme access rather than the
> project or person is much more secure this way.

So to change the subject from GNU OS, how DOES one do this in UNIX?

If I am writing a paper and I want to allow my coauthor, but not the rest
of the world, to edit the file, is there any way to do this without setting
up a new group?