Once you login into your panel account, Go to Login >> Invite A Friend (or similar link as name can be different). Enter the email address under Invite friends labeled text area one email address per line. Click on Invite Friends button, an email will be sent to all the email addresses and they can verify and join the panel.
You can post the tracking URL on your web site/page/blog. You can also directly post the link on facebook, twitter and other social networking sites. Others can join in from this link.
This feature is available with the following license :