adding a db-specific user

I was wandering how I can add a user to a specific db. There are several 
databases but I want to add the user to only one of them. It seems when I 
add a new user, it is entered in the pg_user table and I can log in to any 
one of the other dbs (ofcourse I won't be able to do anything until I grant 
permission still I don't even want to be able to log in to the other dbs and 
see the db schema etc).
Does anybody know how to avoid this?
Thanx
Sally

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