You can upload a list of emails (one per line) in Import Guests. To get them emailed out you need to either set emails to Auto or add auto_send_smtp to the import as well. A hiccup free import would be:
username,email,auto_send_smtp
email1,email1,1
email2,email2,1
email3,email3,1
You can get away with skipping the username column but you need to be careful with the steps and options to ensure the field names are parsed correctly.