Make the name of the checkbox something identifiable to the person, either their name or an id.
<input type='checkbox' name='andy'>
Then when going over the form, have something like if $_POST["andy"] = TRUE, then email andy.
If you want to get the email address from the form to save a trip to a database, use hidden fields.
<input type='hidden' name='emailandy' value='andy@yahoo.com'>