Looking for friends via gmail and subsequent invite?

Question

gmail

How is this opportunity implemented?

Answer 1

Most likely there is an appeal by imap(?), with a list of the user's contacts. Next, you choose who to send the invitation to.

Answer 2

In this particular case, they probably connect to the mailbox via IMAP and scan sent letters.
For gmail and mailru, there are better options - gdata and api.mail.ru (allows you to pull friends out of "my world").
With Yandex.mail, there is another pitfall - the IMAP interface for the user is available only ~ 5 minutes after the first access to it. Tobish, if the user previously used mail only through the web interface, it will not work to import contacts. Only if you ask the user to press the button again after 5-10 minutes :-)