Click "Contacts" in the Sidebar, then click "View / Search".
You can enter a keyword to search for in the "Enter Keyword" box at the top of the page. For example, if you wanted to find all gmail contacts, you would type gmail into the "Enter Keyword" box, then click "Search".
You can add or remove these contact from lists or groups by using the buttons at the top of the search. You can also export the contacts or delete the contacts with the options available at the top of the search.