Why not contact David's - the contact info is on his site and ask them directly why they need your number and what they do with the info? The process for both owners and renters is described in detail on his site.
AFAIK, they do NOT access your membership. They contact you to make any reservations.
Scroll down to the bottom of this page (for
DVC owners interested in using his service) to find the contact info:
http://www.dvcrequest.com/associate/