In some cases, you might need to cherry-pick testers with specific answers on their screener. To achieve this, simply select the Manual option on the screener creation page.
You'll get a list of the participating testers and their answers. Hit the 'Allow booking' button for the users you'd like to have a session with. After you've allowed the booking, the user will be notified and instructed to schedule a session from the available slots. Please note that since the users might not be available on those dates, you'll probably need to allow more users than you strictly require.