Hackers like low-hanging fruit tricks to get victims to click on malicious links. One of the tricks includes sending the victim an email made to look like it comes from Google Groups. Here’s how it works:

First, one doesn’t need a gmail account to be added to a Google group. All the hacker needs to do is associate the victim’s non-gmail address with a gmail account so even if the victim doesn’t have a gmail address, the address will get added to a group. So next time you receive an email that you have been added to a Google Group, someone created a gmail account and associated it with your non-gmail account – and without your knowledge.

Here is the methodology described on Google Groups Support page:

Here is an example of the email sent to the victim: