1. Just hit Compose button or hit reply/reply all button to start mailing.
2. Start typing in the name of the contact you want to mail. Just don't worry whether you have the contact in your address book or not, just type in the first few characters of the first name or the last name.
3. All the matching contacts from both your address book and your Google Plus friends shall be dropped down. You just need to choose the intended contact from the matching contacts.
The integration of Google Plus with Gmail is so good that you will not know whether you are mailing a contact already present in your Google contacts or it has been picked from your Google plus profile.