you dont. You create unix users - postfix just uses them. For virtual domains you follow the workaround tutorial and continue asking your doubts in their list.
suppose i never had a database, Then how wud i add a domain and user to my domain ... I n sendmail we have a a simple comman to crea a user : #cadduser -n mail.domain.com -n username -e username -q 25
how wud be one create it in a similar fashion for postfix ??