users & groups

Hi there,
i have few questions in users and droups in pgSQL that
i can't check in the next few days and this is
inportant in my choice of database:
  1. If I'm using different password files for each
database can I grant or revoke privileges to this
users without creating the same users in pg_user
table? I mean is that enough to create the user only
in password file and to operate with that user,I think
I can't ALTER USER, but what about GRANT ,REVOKE?
  2. How can I add a user in more than one GROUP? How
can I add an existing user  to a new group? How can I
remove a user from some group? What if the users exist
only in password file and not in pg_user table(if it's
possible :-))?

Thanx in advance!
Rumen

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