Why you can no longer have an 'invite-your-friends' feature
Marketing people absolutely love having an 'invite your friends' feature on your website, where your existing users can stick in the email addresses of their friends to have you send invitations to your service to them. Unfortunately, there are a number of problems with it that mean you can't have one any more.
First, if you provide any way to have user-entered text (even in the form of the sending user's nominal real name), spammers will exploit it. Second, even with no user entered text at all it is still going to annoy at least some of the recipients; in fact, you will be sending spam by the UBE definition, since it is email, it is unwanted by at least some recipients, and you will be sending it in bulk.
For the sake of both your reputation and your marketing, it is much better to persuade your users to send personal email messages from their own accounts. If you want to help them out (and increase the chance that they'll actually send the message), you can supply cut and paste ready messages that they can just dump in their mail client, with personalized links and everything.
Sidebar: the distrust problem
Merely having an invite-your-friends feature will cause some people to distrust you for reasons that boil down to 'the well has been poisoned by spammers'; too many untrustworthy websites have kept such email addresses and later used them for additional purposes (marketing, selling them as valuable assets, or whatever).
It follows that if Marketing absolutely forces you to have an invite your friends feature, you should under no circumstances actually keep a copy of these email addresses (or at least not a usable copy; use a hashed version for anti-abuse purposes). And you should have a clear and prominently mentioned policy about it.