[CentOS] Permission problem

Hi,

I have to solve an apparently simple permission problem, and I don't 
know if it's the sunny weather or birdsong, well... I just can't figure 
it out. Here goes.

I have a bunch of users in a public library. Some are "administrators": 
they handle the library, write the docs for everyone, etc. Then you have 
the "agents", who take care of lending books and fetch them when they 
get returned. And then you have the odd anonymous user, using the PC 
with a guest account.

Let's make thing simple and start out with one machine. Every single 
user has a /home/<user> directory. Plus, I added the following directories:

/home/pub

and

/home/echange

Then, I created two groups, "administrators" and "agents". Here's what 
I'd like to achieve (but I think my IQ is just below the required limit 
:oD):

1) Members of the "administrators" group have unlimited read/write 
access to /home/pub and below.

2) Members of the "agents" group have read-only access to /home/pub and 
below.

3) All the others (that is, members of neither "administrators" and 
"agents") have no access at all to /home/pub, not even for listing the 
directory content.

The thing is: I can't seem to formulate my problem in terms of 
user/group/others, as there are no owners, but two distinct groups 
involved.

Any idea how to crack that nut?

Niki