In "Mail" hit "New," click the "To:" link, and select your group. If you click the "+" sign to the left of the group name which appears in the "To" field, it'll list the addressees individually if you want to double-check.
Or from within "People," click on the filter above the list of names on the left (it shows "All" by default), select your group from the dropdown list, and then "Send mail" from the bar at the top.
I agree it's not intuitive at first, but it's really not that hard to work out, unless I've misunderstood what you're trying to do.