The can-spam laws mainly state that you have to disclose that an advertisement is an advertisement if it is sent by email, and that you give your recipients an option to unsubscribe or opt out from your marketing list.
The bigger issue is getting blacklisted for your mail server, domain, etc.
This is why you see services such as constant contact, awebber, mailchimp and so on require that you have mailing lists that actually want to receive your emails.
The good news is that with an invite your friends feature you should not have tons of emails going out where it would be conceived as spam. I would also look into programming the invite your friends feature through a FB plugin where you could post something to your member's friends wall.
The facebook API has changed a lot since I last looked at it, so i dont know if this is possible. I know you can at a minimum have it update the subscribers wall.
Think of some more creative ways to do invites, and take a look at how sites like facebook and linkedin do it for guidance.