I can never remember how to add users to Postfix, mostly because I don't have to do it often enough to commit it to memory. Here it is...hopefully I can remember where I wrote it down next time...
These instructions assume a new user named 'new_user'. Credit to falko and his post on howtoforge for part of this code.
Create the new Unix user, and assign him to the 'users' group
root@jupiter:/home# useradd -d /home/new_user -g users new_user
Give the new user a password in the system
root@jupiter:/home# passwd new_user
Actually create the 'home' folder for your new user (/home/new_user)
root@jupiter:/home# mkdir new_user
Create the 'Maildir' folder inside your user's home folder
root@jupiter:/home# mkdir new_user/Maildir
Change the ownership of the new home folder to 'new_user' in the 'users' group. The '-R' switch tells this command to change ownership of all the contents of the /home/new_user too!
root@jupiter:/home# sudo chown new_user:users -R new_user