Users send emails to themselves and it's delivered to a group of people?

They put the recipients in the bcc box. If you do not see a bcc box when you create a new email, go to Options and there should be an option to show the bcc field.

When you send an email to a person in the bcc box, nobody else can see that you included that person as a recipient (in the old paper days, this was done on paper memos and means "blind carbon copy").

If all your recipients are in the bcc box, Outlook may complain that there is nobody in the To box. So people put various things up there to make Outlook happy, including sending it to themselves.