I have a spark feature request.
Although more spark-savvy users will know to right click a spark contact group to broadcast a message only to that particular group I believe that there could be a broadcast button on each contact group to make this task even easier to the not so spark-savvy users.
Or another idea would be to have a dropdown when clicking the Broadcast button at the top of spark and for it to allow you to select the groups to broadcast the message to.
just a thought