I have a group called
users. It has many users.
Now I want every user in this group to have their own mailbox. So, for example, if
john is such a user, then
firstname.lastname@example.org is an e-mail address.
How can I accomplish this?
In a default postfix setup every mail to $USER@$HOSTNAME will be delivered to $USER. You don't need to set that up explicitly.
For a basic postfix setup this looks like a decent howto: https://help.ubuntu.com/community/PostfixBasicSetupHowto