Make Your Product Useful For Groups

This is the best and easiest kind of referral, and hence the best and easiest kind of growth. If you can think of why your product is useful to more than one person, i.e. Dropbox shared folders, or Expensify invite-your-manager, that is the most natural, non-intrusive way to be referred.
Dropbox created incentives for sharing the service, but the part of their growth that received little attention was the concept of Shared Folders. When an early adopter wanted to use Dropbox with a colleague, they'd invite them to a folder, and have a reason to make sure they joined (for their own selfish convenience).

Look at Rypple.com or Expensify's signup process. Same for 15Five.com.

Your end goal is to have a product useful not just to one person using it alone, but two, or even 50 using it together.

According to Drew Houston of Dropbox, 20% of their user growth comes from shared folders alone. Source: //fr.justin.tv/startuplessonslearned/b/262672510