How do I only allow one group access to my database

I can't seem to understand permissions in PostgreSQL. Can't I set
permissions for an entire database, so that only some defined groups
has access to this database? Do I really need to set permissions for each
table in the database, or how do you guys do it if you have a group of
users that need the same access to some databases?

--
Med venlig hilsen/Best regards,
Søren Neigaard mailto:neigaard(at)e-box(dot)dk
--
"There is a theory which states that if ever anybody discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another theory which states that this has already happened."