When you set up a referral system for your site (including but not limited to dropbox-style schemes that involve referring a certain number of people for a reward) you must let your user write or vet 100% of the copy that gets sent to their friends. You must also tell them exactly how you will be contacting their friends. Will you tweet for them? Will you ever send mail or tweets or facebook messages from their account without them having clicked a submit? You’d better not.
Let your users do the explaining themselves. They understand exactly how to frame your service for their friends, and you don’t. One of the motivations for sharing stuff on the internet with friends is showing off that you know about neat stuff on the internet, and if you make your users look like jerks by limiting them to your prewritten referral messages, oftentimes, they just won’t hit send.