Why do people I did not add appear in my Friends list?

Each time that you send a message from Google Mail, the recipient's email address is automatically added to your Google Mail Contacts list. If you communicate with that person often enough, he or she is automatically added to your Google Talk Friends list so that you can easily communicate, whatever the application you are using. Even if your friends do not have Google Talk, you can still email them by clicking the envelope icon next to their names in your Friends list. Or invite them to talk to you!