When composing Gmail conversations, the "auto-complete" feature allows us to see our matching personal contacts as we type and quickly make our contact selections. This time-saving feature can be duplicated when creating Google Apps Script applications. For instance, if you design an application that requires sending emails, you can leverage this auto-complete feature by using a personal contact list.
As an added feature if the email isn't formatted correctly, then the invalid email is ignored and not selected.
Apps Script has the ability to create applications which allow you to integrate various Google services while building complex user interfaces.
You can view Dito's Email Auto-Complete Script demonstration below:
You can also find Dito Directory on the Google Apps Marketplace.
[Source: Google Apps Developer Blog]