Use invitations when you know the identity of the people you want to respond to your survey. Click on "invitations", then "add".
Important: Remember to set up your email server first. See setup

From the invitation list screen you can see status for all invitations and the send date. You can also change invitation status, view invitees and delete invitations. Invitation can have one of the following statuses:
Sending. Invitation has Sending status if it is actually sending emails at the time you are watching the invitation status.
IMPORTANT: If you deactivate invitation now, it will immediately stop sending emails. However, current execution will still be counted as finished. Thus, if you deactivate invitation which supposes to send email to 1000 invitees while it's running, invitation status will be set to Completed even if it has only sent emails to 100 invitees.
You can change invitation status from this screen. Note that you can't activate invitation if invitee import is in progress.