Maybe this link can help you

mindsharpblogs.com/.../853.aspx

Unfortunately, since I'm trying to do this with the
built-in "user information" and not an actual list or column  from
another list, I don't know how to use this to help me. I am
thinking, silly as it sounds, that it would be good to just send out
a link and ask folks to click on it so they become "site visitors."

There is another way of doing this, add the users programmatically to the WSS site. They will then show up in the lookup  field. Later delete them programmatically.

I have done this before by querying UserInfo table (I know not a good practice to query SharePoint db), and added all the users with Contributors rights to a WSS site on a single click of a button. Another button was to delete all users.

You can create a similar tool using windows application.

It sounds a bit cumbersome, but since there's no built-in
option, this will work for me! I won't do the query thing, but I can
easily add the people I know I need in the lookup  column, and once
we publish and advertise the list, others will hit the site, become
users in the lookup list, etc. Whenever I run into a problem w/ one
or two users not showing in the lookup options, I'll do the
add/delete thing.