Someone else would be better to answer this as I rarely use gmail.
Normally contacts can be initialed using abbreviations (a 'short-name') such as a first or last name or even a nickname in your address book. Just typing in those abbreviations should link them to the address book email address.

In my browser if I type in the first one or two letters of the 'short-name', then the browser will suggest which short-name (email address) I wish to enter with a drop-down menu of short-names so I click the suggested short-name. That makes it oh so quick to send an email to multiple persons in your address-book. You may need to edit your contacts list (address book) to give them a short-name.