newbie authorization question

[Marisa Brandt]

I have what I think is probably an ultra-basic question:

I'm authenticating users via ldap; everything appears to be working 
perfectly.  However, I'd like to restrict access to all users, EXCEPT 
for a few select groups.  I'm clearly missing something really obvious, 
because I can't seem to figure out how to do that.  Can someone help me out?

Thanks very much,
Marisa