If you need to contact someone frequently who is not a member of either the CUC or PAA domain you may want to add them to your contacts list. Anyone who is a member of CUC or PAA is already accessible through auto fill so you do not need to add them to your contacts list.
When you are on the Contacts screen you can select Add to "My Contacts" and enter the email address directly in the dropdown box. Make sure to click Add when you are finished.
The contact is now ready to use, but you can add further detail if you like by selecting the contact out of the list. You can add a picture, name, phone numbers, etc. With Gmail you can personalize the contacts as much or as little as you like.
Now Gmail can auto complete the address for you when you send a message.