Re: CF8 & AJAX to populate form fields
as Dan is suggesting, use what is usually called 'autosuggest' to help
users find the user they want by letting them type in first 2-3
characters of the name and showing them a short list of users with names
starting with those characters, instead of having them scr0ll through
4500 list items to select a user.
after that you do not really need to use ajax to populate your form
fields - instead have a form in a different page and relocate the user
to that page passing the id of user selected in the autosuggest list.
this is easier than using ajax to populate various form fields, though
if you are on cf8 it is fairly trivial, too with data bindings or cf