Tagged Google Contacts Retrieval

[Grady Laksmono]

Hi, I looked at Tagged.com and I liked the way they could retrieve the
contact information from Google, Yahoo, Live, etc.. but I'm wondering
on how could they achieve that?

Here's the screenshot of the invite page:
http://i50.photobucket.com/albums/f322/Monyet88/Untitled-1-1.jpg

As soon as we press the "Find" button, it will retrieve the contact
information from the email that is specified there.. The password is
using the email's password.. In this case, if I want to retrieve
contacts from Gmail using Google Contacts API.. Anyone could give me
direction/suggestion to approach this?

Thanks!