Click on the Account tab.
Then click on the Contacts tab and under Connect with Google or Outlook click on the appropriate button to connect with that service.
Once you have connected, the button will change to Disconnect.
When sending a document and you are on the Add Participants screen, as you type a name or email address you will now see contacts from Google or Outlook matching the text you are entering. This allows you to "search" for a contact to add as a participant.