Hi,
I think it'll be easier to let the persons who were sent the pdf to sign insert themselves their e-mail address when they'll be signing.
Why do this?
When we need to send a document to be sign by more than 50, 100 or even 200 persons (when we need to check the presence of people at a webinar event, for instance) it's too long to do it now in Adobe Sign. We need to select and drag a signature field for each and everyone of them.