If you need to add additional contacts, you can do so by searching for your contact's name in the search field at the top of the window. Simply type in the name of your contact and hit the enter key.
The screen will change and your contact's card should pop up. If it does not, you can force OWA to search the entire UI directory by checking the directory radio button on the side of the page.
To add the contact to your list, click on their card and then click the add user button to add the contact. You can either add them to your full email contacts or Lync contacts.