Uninvited contacts appear in my Friends list

Each time you send a message from Gmail, the recipient's email address is automatically added to your Gmail Contacts list. If you communicate with that person often enough, he or she is automatically added to your Google Talk Friends list so you can easily communicate, no matter which application you're using. Even if your friends don't 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!