You can pass bargain to you users. Here are some examples:
a) A refer a friend programme (paid in groupon bucks)
b) Multiple purchases – say tickets for a friends
c) Daily e-mails with the latest bargains Example Justin Jackson Email Tiny Marketing Wins:

“Your discount is expiring soon

Time is running out to use that $70 discount I sent you. If you want help with your marketing, now’s the time to sign up:

Register here
Use todayistheday70 to get a year of TMW for $300 $229
You’ll get to implement a new tactic every week + ask me your marketing questions.”

Awesome List of referall programms:

https://www.referralcandy.com/blog/47-referral-programs/