It's quite simple really, and as far as I know doesn't require Procmail configuration. Here's what I did.
1. Create user accounts for everyone you want to have access to mail on the system that is running Sendmail/IMAP.
2. Configure Mail.app for each user on which ever systems they use. Set the incomming mail server to the system you have sendmail/IMAP running on. NOTE: Unless you edit each computer's NetInfo Machines Directory and Lookup order, just use the IP address of the computer.
3. That's it. This worked for me with no problems, other than the one I posted for this article.
4. Make sure to follow the steps for 'Allowing Relaying from Certain Hosts' in this article so these other users can send mail.
Hope this helps.