Addressing a message in Gmail is similar to other applications. You can click in the To box and start typing the recipient name or can click on the To link to find contacts in the corporate directory or your own contact list. 

To send a message to a contact or a contact group, simply type the name of the contact or group to the right of the To box of the new message. All matches within your contact list and the Corporate Directory will be displayed: 

If you type a group name, all email addresses in the group will be added to the To box when you press Return.

You can choose which contacts to address a message to as follows:

If you often send messages to the people you have ticked in the contact list, click on the Manage Labels button, then on Create Label to save the people into a contact group. You will be prompted for a label name - enter a name and click on Save.