How to prepopulate form fields with email address

I have a form where in two different places it asks the user to populate the field with someone elses' email address. Rather than the user having to type the full email address for every submission, i would like the field to prepopulate from a list. I understand that creating a contact list in the relevant fields in the Sheet, will not be presented in the form. Is this correct? So is my only option to change the form field to a drop down list and enter all the email addresses in that list? if so, how do i get this list to sort alphabetically, and secondly, how do i provide the user with the option to enter an email address that is not in the list, ensuring, that the email that they entered manually, ends up in the correct field in the sheet. Also, how can i automatically update and sort the list (alphabetically) each time a user adds a new email address.

Thanks

Karen

Answers