Creating e-mail distribution lists
On the menubar click on Tools and choose Address Book. In the new window, click on New and enter a nickname for the group. In the box below enter all the nicknames or e-mail addresses you would like the group name to expand into.

Using e-mail distribution lists
Open a new message composition window. Besides To type the name of the desired group. This will expand into all the e-mail addresses when you click on the send button.