[tlug] group question

[Jean-Christian Imbeault <jc@example.com>]

I'm having trouble figuring out groups on my RH7.2 machine.

I want to have two users, A and B, and give B the ability to read, 
write, delete files/directories that belong to user A. While A can only 
read B's files but not modify them.

I tried adding B to the same group as A:

# cat /etc/group
A:x:502:A,B
B:x:504:B

but that didn't work. In a directory with the following perms, A was 
able to delete files created by B:

drwxrwxr-x   59 A	A

even though the file had these perms:

-rw-r--r--    1 B	B

But strangely enough A couldn't *overwrite* files created by B.

I've gone through two Linux sysadmin books, and a few online refs and it 
seems that what I want to do should be done by adding B to A's group. 
But it's not working so obviously I am misunderstanding the group concept.

What is the proper way to give one user the ability to have the perms of 
another user using groups?

Jc

PS I am trying to do this for user who only have FTP access, so su/sg or 
other command line commands are not an option. I want to give user B the 
ability to fiddle around with A's files ...