As someone who has rented points numerous times before buying into DVC, I always called to confirm my ressie the minute I received it in the mail (and before I sent the person money). I am 99% sure that there was a phone number listed on the confirmation letter and if I recall, it was members services. I just called the number and said I needed to confirm a ressie. I have never been asked for a membership number. Also, I have never, ever had a problem renting points from anyone on these boards. Everyone has been wonderful that I rented from.
As a side note, when I have rented in the past, most members asked me to "never say you rented points. If anyone asks, just say that you are a guest of a member". That being said, our guide told us "you can rent points any time you wish". While I don't intend on doing that (I bought so we can go on vacation), I was a little surprised that she said that.
Lisa