Yes. Go to Guest List. To display all guests, including those who have not yet responded or registered, click the Guest Filter icon and select:
View All Invited
This will load all guests invited to the event.
Find the guest using the search bar.
Select their row using the checkbox. Then click the green thumbs-up button in the toolbar.
This will accept their invite on their behalf and set them as confirmed. You can also decline on behalf of a guest using the same method with the red thumbs-down button.