To send invites to unregistered users, do the following:
Go to Email broadcast > Subscribers list and create a new list or go to an existing subscriber list.
Click on Import to import email addresses and their first and last name.
Once subscribers are imported, go to Email broadcast > Email campaigns
Create a new campaign and enter your welcome message or choose a preset email template. Select the subscriber list from the list of Recipients.
Click on Send Campaign on the top right to send the campaign.