Getting into an invitation only system on a new service or social site is sometimes considered as a "status symbol". And folks want to proudly declare "I have a Google Plus account, I am so special". It's human nature, I think.Google seems understand very well that things and used it well too when launched Google+ service with an invitation only. People are going crazy for a Google plus invite in early weeks of its release.
Unfortunately this situations is exploited by some peoples who are not responsible. Mostly done by spammers and some professionals to make their business out of it. They are creating a fake email template which exactly resembles the invitation email of Google Plus and they are sending the fake invitation email to random users from an email list they bought.
Here is how the fake email invitation of Google Plus looks like [courtesy: Naked Security]:
