This should be quite basic but I don't have a clue how to do this.

If've got a file that I need to give specific permission to various people (ie not one group) to access it , only they my access it, no one else.

What must I Do

Hi,

This is one way:

Create a new group with groupadd,
give 'the file' this group,
change group attributes (if needed),
add group to all users that need to access this file.

I.e.:

File in question testfile, example user is johndoe, newgroup xs4all:

$ groupadd xs4all
$ chgrp xs4all testfile
$ chmod 664 testfile
$ usermod -G xs4all,<other>,<other> johndoe

Do read the usermod manpage about the -G flag!!!!

If you are happy with the original group the testfile has, you can skip the first three and only do step 4 (adding the group to the user(s) using usermod).

All actions should be done by root.

Hope this is clear enough, if not. Just say so.