The fake invites are spreading through email, and they look very similar to the actual invites to the service, down to the email address which seems to be coming from a Google+ member.
However, if you click on a link in the message – which we definitely do not recommend – you’ll land at a site trying to send you penis enlargement products.
Scams such as this one are very common with major product launches; most recently, we’ve seen a similar phishing scheme involving Google Music invites. Whenever you receive an “invite” to a service, especially from a third party, be extra careful before clicking any links and make sure that the invite is really coming from a legitimate source.