How to hack someone's Gmail account

Someone recently asked me about whether they might have had their Gmail account hacked into, and in investigating the answer, I realized that there's an elegant, if somewhat tricky way you can invite someone to join Google's popular Gmail service in such a way that it's quite likely you'll later be able to shut them out and hijack their Gmail account. Surprised? I sure was, so it's fortunate that I also explain how to check to see if your account might be compromised and how to fix it:

    How can someone hack my Google Gmail account?

Again, if you have a Gmail account and aren't sure you got the original invitation from someone legitimate, please check through this article to ensure you aren't at risk.