There are two parts to this:
1) On the server, load the POP3 Server task with "load pop3"; you can also add it to your "TASKS=" line in your notes.ini file if you'd like.
2) Create a person document for the user and fill in the HTTP/Internet password. The user will have to log in using their user name (fully qualified if you specify a Notes domain) and this password in their POP3 client.
Note that even if these users do not have a Notes ID, there is a Lotus per user charge of $30 for each Domino/POP3 mail user.