There would be three main differences between my example and what you want to do that would take some testing and playing.
1) Getting the value of the field were you select the user, assuming that you’re using a user field.
2) How you populate the email field, assuming you’ll be wanting to use an email field
3) The php function that gets and returns the email will need to look at users instead of posts.
If I have some time in the next few days I’ll try to take a look at it.
