The email action in SharePoint Designer is one of the commonly used actions. This is basically for being notified by email in case an event occurs.
Once you edit the action for an email, one would observe fields which you would find in any email client. Against the To field there is a small icon which can be clicked. Once you click that Icon, you would observe a list of users and Groups. Now the key thing to note is that these list and Groups are obtained from SharePoint only.
The reason why I say that it is important is because in the Office 365 Admin center also has groups which can be created. One could assign a group email and add members to it. But unfortunately this doesn’t work in SharePoint Designer since it only gets information from SharePoint. So in order to email to a group from SharePoint Designer, one would have to create a group in SharePoint by going to site settings.