If you want to send an invitation to someone you've sent to in the past, you can use the Past guest lists option to see a full list of older contacts.
Once you click this link, you'll be shown all the previous invitations you've sent out. Click on an invitation to see the email addresses for that event.
You can then add individual contacts by clicking the check mark next to their name or use the Select all button to add all of the contacts at once. Once you're done, click Add to list to finish up.