Newbie: How to add a new field from a drop-down

Newbie: How to add a new field from a drop-down

So I have people submitting abstracts using a form. The abstracts may have a variable number of authors. I've created a drop-down that says "Add Second Author" "Add Third Author" etc., and want a new name window to open up in the form so people can then enter additional author names. How to do? Thanks.