There is a workflow that is sending email action but it is not sending alert notification for "Assign a task" to a SharePoint group. It is a SharePoint designer 2013 workflow built on SharePoint online list. The workflow runs on item created.
By default, we can't send the mail to all users those are present in the group. If the SharePoint Group or Ad Group has the Group Email, then the email sent the mail to group email Id.
If you want the email individually means, just iterate the user and send the email to users.