From the Zimbra Help tab;
Creating group contact lists
The Contact Group List feature allows you to create contact lists that contain multiple mailing addresses. When you select a group contact name, everyone whose address is included in the group list is automatically added to the address field of the message.
To create contact group lists
You can select addresses from your company directory (GAL) and from your address books, and you can type in addresses. Note: email addresses you type are not verified.
On the toolbar, click New and select New Contact Group. The New Contact Group form is displayed.
If you want to create the group list in an address book other than Contacts, before you start, select the address book and then click New>Contact Group.
Note: You can move a contact group to another list after it is created.
In the Group Name field, type the name of this group list.
To add members to this group, in the Find field, type a name and then select whether to search in the GAL or in your personal and shared contacts. Click Search.
Names that match your entry are listed in the box below. Select the names to enter and click Add, the names are added to the Group Members list.
You can also add email addresses directly in the Group Members list box. Typed addresses can be separated by a comma (,), semicolon (
or Return to start the address on a new line.
Click Save on the toolbar. The new group contact is added to your address book.
The icon groupcontasticon.png shows that the contact name is a group contact.