Gmail uses Google Contacts as its primary address book, from which you can add the recipient's contacts to any new email.
Google Contacts can be used not only in Gmail, but also in the phone book on Android mobile phones and other Google services. For example, if you set a person's date of birth in contacts, the Google Calendar (which is by default in Google Account together with Gmail mail) will automatically add an event that will remind you of the birthday of a new contact in advance. And if the contact you're adding already has a Google account with a photo in his or her profile, it'll automatically show up and make it easier to understand who's writing to you.
Create contactbutton in the top left corner.