The ability to enable the Refer-a-Friend entry option feature is available by upgrading to the premium subscription. This entry option will allow your entrants to receive additional entries into your giveaway by encouraging their friends to visit and enter the giveaway. This helps motivate your entrants to share your giveaway, spreading the word.
If you check the “Enable Refer-A-Friend (aka. 'viral sharing')” button when creating a giveaway, here’s what will happen: after one of your entry options has been completed by the entrant, a 'Bring Your Friends!' entry option will appear on the widget that will give each entrant a unique URL that they can share with their friends. For each friend that enters the giveaway through that unique URL, the referring individual will receive +1 entry into the giveaway. Each entrant can refer up to 10 people for each giveaway.
You'll be able to track who each entrant referred in your page's entries grid as well as an overview of how many total entrants were referred to your giveaway on your Rafflecopter Analytics dashboard.
Below is a screenshot of a widget showing this entry option: