Hi Sarah!
Based on feedback I have received from merchants in recent times, they have spoken very highly of the ReferralCandy app:
https://apps.shopify.com/referralcandy
When a customer buys something from your store the app sends them a coupon code to share with friends. When their friends buy from you, the customer receives a referral reward.
Hope this helps!
Andrew
Shopify Guru
support@shopify.com