How to retrieve email addresses from my gmail account?

I am writing an "Invite Users" feature for my website.

I wanted the user to enter his开发者_运维技巧 gmail credentials or facebook credentials, and I wanted to send emails to the friends he chooses from.

Although I found out that you cannot send privates messages to your friends on FB via the API.

How can I accomplish the same using google / gmail ?

You should use the Google Contacts API along with OAuth. Accepting the user's GMail ID and password on your own site is never a good idea. http://code.google.com/apis/contacts/