If you are running an event, have a subscription product or some kind of paid access digital product, offer customers the chance to get it for free if they refer two or more friends. Then extend the same offer to each of those referred friends to generate a viral exponential growth loop. This is great for when you need to build up volume and aren't too concerned with revenue just yet. Dropbox used this technique to great effect in its early days. You can use a tool like the Viral Coefficient Calculator from Nichevertising to calculate how many referrals to ask for based on your open rates.