Do you have a user that already gets mail?
If so, look in their home directory for either 'mail' or 'Maildir' depending on if you are using mbox or Maildir to store your mail. Then emulate that directory structure in you other user's directories and make sure your users own those new directories.
If you are using mbox I think you only need to create a 'mail' folder in their home directory. If it is Maildir, you need to create a Maildir directory containing a cur, new, and tmp directories.
Sendmail does not need to know what all users you have, it just gets an email to
user@example.com.
-checks that it is supposed to get mail for example.com
-checks if there is a user called user on the machine.
-checks that user has directory to put mail.
-delivers mail.